Merge branch 'develop'v0.23.03

97 files changed, 12089 insertions, 1235 deletions

diff --git a/.gitignore b/.gitignore
index ba88570..c32d36b 100644
--- a/.gitignore
+++ b/.gitignore
@@ -22,6 +22,7 @@ build/
 
 # doc builds
 doc/build/
+doc/source/api
 
 # doc extra files
 
diff --git a/.pylintrc b/.pylintrc
index 7885c4e..418a728 100644
--- a/.pylintrc
+++ b/.pylintrc
@@ -76,10 +76,10 @@ max-attributes=7
 max-bool-expr=5
 
 # Maximum number of branch for function / method body.
-max-branches=15
+max-branches=20
 
 # Maximum number of locals for function / method body.
-max-locals=15
+max-locals=20
 
 # Maximum number of parents for a class (see R0901).
 max-parents=7
@@ -88,10 +88,10 @@ max-parents=7
 max-public-methods=20
 
 # Maximum number of return / yield for function / method body.
-max-returns=6
+max-returns=15
 
 # Maximum number of statements in function / method body.
-max-statements=50
+max-statements=100
 
 # Minimum number of public methods for a class (see R0903).
 min-public-methods=1
@@ -144,6 +144,19 @@ allow-wildcard-with-all=no
 logging-format-style=old
 
 
+[MESSAGES CONTROL]
+
+# disable similarities check
+disable=raw-checker-failed,
+        bad-inline-option,
+        locally-disabled,
+        file-ignored,
+        suppressed-message,
+        useless-suppression,
+        deprecated-pragma,
+        use-symbolic-message-instead,
+        duplicate-code
+
 
 [MISCELLANEOUS]
 
@@ -164,7 +177,7 @@ score=yes
 [SIMILARITIES]
 
 # Minimum lines number of a similarity.
-min-similarity-lines=4
+min-similarity-lines=5
 
 
 [STRING]
diff --git a/CHANGELOG.md b/CHANGELOG.md
new file mode 100644
index 0000000..fb66c1e
--- /dev/null
+++ b/CHANGELOG.md
@@ -0,0 +1,28 @@
+
+# Changelog
+
+## 0.23.03 (Initial release)
+
+### Added
+
+- File graph protocol
+  - Graph access and navigation
+  - Syntactic sugar
+  - Fetch result shortcuts
+- Filter and Fetch Queries
+  - Syntax trees
+  - Validation
+  - Matching
+- Infrastructure to Open a storage
+- Storage schema
+- Backend
+  - Basic interface
+  - Sparql triple store: Manage triples via rdflib and sparql.
+- Access controls
+  - Basic interface
+  - NullAC: A dummy access control mechanism.
+- Essential utilities
+  - URI
+  - uuid
+  - namespaces
+
diff --git a/MANIFEST.in b/MANIFEST.in
new file mode 100644
index 0000000..a06c41c
--- /dev/null
+++ b/MANIFEST.in
@@ -0,0 +1 @@
+include bsfs/graph/schema.nt
diff --git a/README b/README
deleted file mode 100644
index da066f6..0000000
--- a/README
+++ /dev/null
@@ -1,57 +0,0 @@
-
-The Black Star File System
-==========================
-
-
-### Developer tools setup
-
-#### Test coverage (coverage)
-
-Resources:
-* https://coverage.readthedocs.io/en/6.5.0/index.html
-* https://nedbatchelder.com/blog/200710/flaws_in_coverage_measurement.html
-
-Commands:
-$ pip install coverage
-$ coverage run ; coverage html ; xdg-open .htmlcov/index.html
-
-
-
-#### Static code analysis (pylint)
-
-Resources:
-* https://github.com/PyCQA/pylint
-* https://pylint.org/
-* https://pylint.pycqa.org/en/latest/user_guide/messages/messages_overview.html#messages-overview
-
-Commands:
-$ pip install pylint
-$ pylint bsfs
-
-
-
-#### Type analysis (mypy)
-
-Resources:
-* https://github.com/python/mypy
-* https://mypy.readthedocs.io/en/stable/
-
-Commands:
-$ pip install mypy
-$ mypy
-
-
-
-#### Documentation (sphinx)
-
-Resources:
-* 
-* 
-
-Commands:
-$ pip install ...
-$ 
-
-
-
-
diff --git a/README.md b/README.md
new file mode 100644
index 0000000..796c198
--- /dev/null
+++ b/README.md
@@ -0,0 +1,59 @@
+
+# The Black Star File System
+
+The Black Star File System (BSFS) is a semantic file system, meaning that it organizes files
+by association, and can record files, their metadata and content in a structured manner.
+
+
+## Installation
+
+You can install BSFS via pip:
+
+    $ pip install --extra-index-url https://pip.bsfs.io bsfs
+
+
+## Development
+
+Set up a virtual environment:
+
+    $ virtualenv env
+    $ source env/bin/activate
+
+Install bsfs as editable from the git repository:
+
+    $ git clone https://git.bsfs.io/bsfs.git
+    $ cd bsfs
+    $ pip install -e .
+
+If you want to develop (*dev*), run the tests (*test*), edit the 
+documentation (*doc*), or build a distributable (*build*),
+install bsfs with the respective extras:
+
+    $ pip install -e .[dev,doc,build,test]
+
+Or, you can manually install the following packages besides BSFS:
+
+    $ pip install coverage mypy pylint
+    $ pip install sphinx sphinx-copybutton furo
+    $ pip install build
+
+To ensure code style discipline, run the following commands:
+
+    $ coverage run ; coverage html ; xdg-open .htmlcov/index.html
+    $ pylint bsfs
+    $ mypy
+
+To build the package, do:
+
+    $ python -m build
+
+To run only the tests (without coverage), run the following command from the **test folder**:
+
+    $ python -m unittest
+
+To build the documentation, run the following commands from the **doc folder**:
+
+    $ sphinx-apidoc -f -o source/api ../bsfs/ --module-first -d 1 --separate
+    $ make html
+    $ xdg-open build/html/index.html
+
diff --git a/bsfs.app b/bsfs.app
index babacbb..c837ca0 100755
--- a/bsfs.app
+++ b/bsfs.app
@@ -1,52 +1,6 @@
-"""BSFS tools.
-
-Part of the BlackStar filesystem (bsfs) module.
-A copy of the license is provided with the project.
-Author: Matthias Baumgartner, 2022
-"""
-# imports
-import argparse
-import typing
-
-# module imports
-import bsfs
-import bsfs.apps
-
-# exports
-__all__: typing.Sequence[str] = (
-    'main',
-    )
-
-# config
-apps = {
-    'init'      : bsfs.apps.init,
-    'migrate'   : bsfs.apps.migrate,
-    }
-
-
-## code ##
-
-def main(argv):
-    """Black Star File System maintenance tools."""
-    parser = argparse.ArgumentParser(description=main.__doc__, prog='bsfs')
-    # version
-    parser.add_argument('--version', action='version',
-        version='%(prog)s version {}.{}.{}'.format(*bsfs.version_info))
-    # application selection
-    parser.add_argument('app', choices=apps.keys(),
-        help='Select the application to run.')
-    # dangling args
-    parser.add_argument('rest', nargs=argparse.REMAINDER)
-    # parse
-    args = parser.parse_args()
-    # run application
-    apps[args.app](args.rest)
-
-   
-## main ##
-
+#!/usr/bin/env python3
 if __name__ == '__main__':
+    import bsfs.apps
     import sys
-    main(sys.argv[1:])
+    bsfs.apps.main(sys.argv[1:])
 
-## EOF ##
diff --git a/bsfs.toml b/bsfs.toml
deleted file mode 100644
index 45bf1c9..0000000
--- a/bsfs.toml
+++ /dev/null
@@ -1,11 +0,0 @@
-[project]
-name = "bsfs"
-description = "A content aware graph file system."
-version = "0.0.1"
-license = {text = "BSD 3-Clause License"}
-authors = [{name='Matthias Baumgartner', email="dev@igsor.net"}]
-dependencies = [
-  "rdflib",
-]
-requires-python = ">=3.7"
-
diff --git a/bsfs/__init__.py b/bsfs/__init__.py
index 079ffaf..cf08d64 100644
--- a/bsfs/__init__.py
+++ b/bsfs/__init__.py
@@ -1,9 +1,4 @@
-"""
 
-Part of the BlackStar filesystem (bsfs) module.
-A copy of the license is provided with the project.
-Author: Matthias Baumgartner, 2022
-"""
 # imports
 import collections
 import typing
diff --git a/bsfs/apps/__init__.py b/bsfs/apps/__init__.py
index 7efaa87..62dc5b5 100644
--- a/bsfs/apps/__init__.py
+++ b/bsfs/apps/__init__.py
@@ -1,20 +1,53 @@
-"""
+#!/usr/bin/env python3
 
-Part of the BlackStar filesystem (bsfs) module.
-A copy of the license is provided with the project.
-Author: Matthias Baumgartner, 2022
-"""
 # imports
+import argparse
 import typing
 
+# bsfs imports
+import bsfs
+
 # inner-module imports
 from .init import main as init
 from .migrate import main as migrate
 
 # exports
 __all__: typing.Sequence[str] = (
+    'main',
     'init',
     'migrate',
     )
 
+# config
+apps = {
+    'init'      : init,
+    'migrate'   : migrate,
+    }
+
+
+## code ##
+
+def main(argv=None):
+    """Black Star File System maintenance tools."""
+    parser = argparse.ArgumentParser(description=main.__doc__, prog='bsfs')
+    # version
+    parser.add_argument('--version', action='version',
+        version='%(prog)s version {}.{}.{}'.format(*bsfs.version_info)) # pylint: disable=C0209
+    # application selection
+    parser.add_argument('app', choices=apps.keys(),
+        help='Select the application to run.')
+    # dangling args
+    parser.add_argument('rest', nargs=argparse.REMAINDER)
+    # parse
+    args = parser.parse_args(argv)
+    # run application
+    apps[args.app](args.rest)
+
+
+## main ##
+
+if __name__ == '__main__':
+    import sys
+    main(sys.argv[1:])
+
 ## EOF ##
diff --git a/bsfs/apps/init.py b/bsfs/apps/init.py
index 3e2ef37..9afbdd5 100644
--- a/bsfs/apps/init.py
+++ b/bsfs/apps/init.py
@@ -1,9 +1,5 @@
-"""
+#!/usr/bin/env python3
 
-Part of the BlackStar filesystem (bsfs) module.
-A copy of the license is provided with the project.
-Author: Matthias Baumgartner, 2022
-"""
 # imports
 import argparse
 import json
@@ -60,9 +56,10 @@ def main(argv):
     # print config
     if args.output is not None:
         with open(args.output, mode='wt', encoding='UTF-8') as ofile:
-            json.dump(config, ofile)
+            json.dump(config, ofile, indent=4)
     else:
-        json.dump(config, sys.stdout)
+        json.dump(config, sys.stdout, indent=4)
+        print('')
 
 
 ## main ##
diff --git a/bsfs/apps/migrate.py b/bsfs/apps/migrate.py
index 91c1661..34ea2e7 100644
--- a/bsfs/apps/migrate.py
+++ b/bsfs/apps/migrate.py
@@ -1,9 +1,5 @@
-"""
+#!/usr/bin/env python3
 
-Part of the BlackStar filesystem (bsfs) module.
-A copy of the license is provided with the project.
-Author: Matthias Baumgartner, 2022
-"""
 # imports
 import argparse
 import json
@@ -42,15 +38,15 @@ def main(argv):
     graph = bsfs.Open(config)
 
     # initialize schema
-    schema = bsfs.schema.Schema.Empty()
+    schema = bsfs.schema.Schema()
     if len(args.schema) == 0:
         # assemble schema from standard input
-        schema = schema + bsfs.schema.Schema.from_string(sys.stdin.read())
+        schema = schema + bsfs.schema.from_string(sys.stdin.read())
     else:
         # assemble schema from input files
         for pth in args.schema:
             with open(pth, mode='rt', encoding='UTF-8') as ifile:
-                schema = schema + bsfs.schema.Schema.from_string(ifile.read())
+                schema = schema + bsfs.schema.from_string(ifile.read())
 
     # migrate schema
     graph.migrate(schema, not args.remove)
diff --git a/bsfs/front/__init__.py b/bsfs/front/__init__.py
index 92886ab..cedcd7f 100644
--- a/bsfs/front/__init__.py
+++ b/bsfs/front/__init__.py
@@ -1,9 +1,4 @@
-"""
 
-Part of the BlackStar filesystem (bsfs) module.
-A copy of the license is provided with the project.
-Author: Matthias Baumgartner, 2022
-"""
 # imports
 import typing
 
diff --git a/bsfs/front/bsfs.py b/bsfs/front/bsfs.py
index 968b3f5..f437212 100644
--- a/bsfs/front/bsfs.py
+++ b/bsfs/front/bsfs.py
@@ -1,9 +1,4 @@
-"""
 
-Part of the BlackStar filesystem (bsfs) module.
-A copy of the license is provided with the project.
-Author: Matthias Baumgartner, 2022
-"""
 # imports
 import typing
 
diff --git a/bsfs/front/builder.py b/bsfs/front/builder.py
index 73f1703..b1d488b 100644
--- a/bsfs/front/builder.py
+++ b/bsfs/front/builder.py
@@ -1,14 +1,9 @@
-"""
 
-Part of the BlackStar filesystem (bsfs) module.
-A copy of the license is provided with the project.
-Author: Matthias Baumgartner, 2022
-"""
 # imports
 import typing
 
 # bsfs imports
-from bsfs.graph import Graph
+from bsfs.graph import Graph, ac
 from bsfs.triple_store import TripleStoreBase, SparqlStore
 from bsfs.utils import URI, errors
 
@@ -68,8 +63,10 @@ def build_graph(cfg: typing.Any) -> Graph:
     if 'backend' not in args:
         raise errors.ConfigError('required argument "backend" is not provided')
     backend = build_backend(args['backend'])
+    # build access controls
+    access_controls = ac.NullAC(backend, user)
     # build and return graph
     cls = _graph_classes[name]
-    return cls(backend, user)
+    return cls(backend, access_controls)
 
 ## EOF ##
diff --git a/bsfs/graph/__init__.py b/bsfs/graph/__init__.py
index 82d2235..8d38d23 100644
--- a/bsfs/graph/__init__.py
+++ b/bsfs/graph/__init__.py
@@ -1,9 +1,4 @@
-"""
 
-Part of the BlackStar filesystem (bsfs) module.
-A copy of the license is provided with the project.
-Author: Matthias Baumgartner, 2022
-"""
 # imports
 import typing
 
diff --git a/bsfs/graph/ac/__init__.py b/bsfs/graph/ac/__init__.py
index 420de01..11b45df 100644
--- a/bsfs/graph/ac/__init__.py
+++ b/bsfs/graph/ac/__init__.py
@@ -1,9 +1,4 @@
-"""
 
-Part of the BlackStar filesystem (bsfs) module.
-A copy of the license is provided with the project.
-Author: Matthias Baumgartner, 2022
-"""
 # imports
 import typing
 
diff --git a/bsfs/graph/ac/base.py b/bsfs/graph/ac/base.py
index bc9aeb3..e85c1dd 100644
--- a/bsfs/graph/ac/base.py
+++ b/bsfs/graph/ac/base.py
@@ -1,17 +1,13 @@
-"""
 
-Part of the BlackStar filesystem (bsfs) module.
-A copy of the license is provided with the project.
-Author: Matthias Baumgartner, 2022
-"""
 # imports
 import abc
 import typing
 
 # bsfs imports
 from bsfs import schema
+from bsfs.query import ast
 from bsfs.triple_store import TripleStoreBase
-from bsfs.utils import URI
+from bsfs.utils import URI, typename
 
 # exports
 __all__: typing.Sequence[str] = (
@@ -43,6 +39,20 @@ class AccessControlBase(abc.ABC):
         self._backend = backend
         self._user = URI(user)
 
+    def __str__(self) -> str:
+        return f'{typename(self)}({self._user})'
+
+    def __repr__(self) -> str:
+        return f'{typename(self)}({self._user})'
+
+    def __eq__(self, other: typing.Any) -> bool:
+        return isinstance(other, type(self)) \
+           and self._backend == other._backend \
+           and self._user == other._user
+
+    def __hash__(self) -> int:
+        return hash((type(self), self._backend, self._user))
+
     @abc.abstractmethod
     def is_protected_predicate(self, pred: schema.Predicate) -> bool:
         """Return True if a predicate cannot be modified manually."""
@@ -67,5 +77,16 @@ class AccessControlBase(abc.ABC):
     def createable(self, node_type: schema.Node, guids: typing.Iterable[URI]) -> typing.Iterable[URI]:
         """Return nodes that are allowed to be created."""
 
+    @abc.abstractmethod
+    def filter_read(
+            self,
+            node_type: schema.Node,
+            query: typing.Optional[ast.filter.FilterExpression],
+            ) -> typing.Optional[ast.filter.FilterExpression]:
+        """Re-write a filter *query* to get (i.e., read) *node_type* nodes."""
+
+    @abc.abstractmethod
+    def fetch_read(self, node_type: schema.Node, query: ast.fetch.FetchExpression) -> ast.fetch.FetchExpression:
+        """Re-write a fetch *query* to get (i.e, read) values for *node_type* nodes."""
 
 ## EOF ##
diff --git a/bsfs/graph/ac/null.py b/bsfs/graph/ac/null.py
index 36838bd..c9ec7d0 100644
--- a/bsfs/graph/ac/null.py
+++ b/bsfs/graph/ac/null.py
@@ -1,15 +1,11 @@
-"""
 
-Part of the BlackStar filesystem (bsfs) module.
-A copy of the license is provided with the project.
-Author: Matthias Baumgartner, 2022
-"""
 # imports
 import typing
 
 # bsfs imports
 from bsfs import schema
 from bsfs.namespace import ns
+from bsfs.query import ast
 from bsfs.utils import URI
 
 # inner-module imports
@@ -28,7 +24,7 @@ class NullAC(base.AccessControlBase):
 
     def is_protected_predicate(self, pred: schema.Predicate) -> bool:
         """Return True if a predicate cannot be modified manually."""
-        return pred.uri == ns.bsm.t_created
+        return pred.uri == ns.bsn.t_created
 
     def create(self, node_type: schema.Node, guids: typing.Iterable[URI]):
         """Perform post-creation operations on nodes, e.g. ownership information."""
@@ -49,4 +45,16 @@ class NullAC(base.AccessControlBase):
         """Return nodes that are allowed to be created."""
         return guids
 
+    def filter_read(
+            self,
+            node_type: schema.Node,
+            query: typing.Optional[ast.filter.FilterExpression]
+            ) -> typing.Optional[ast.filter.FilterExpression]:
+        """Re-write a filter *query* to get (i.e., read) *node_type* nodes."""
+        return query
+
+    def fetch_read(self, node_type: schema.Node, query: ast.fetch.FetchExpression) -> ast.fetch.FetchExpression:
+        """Re-write a fetch *query* to get (i.e, read) values for *node_type* nodes."""
+        return query
+
 ## EOF ##
diff --git a/bsfs/graph/graph.py b/bsfs/graph/graph.py
index b7b9f1c..1b4c212 100644
--- a/bsfs/graph/graph.py
+++ b/bsfs/graph/graph.py
@@ -1,20 +1,18 @@
-"""
 
-Part of the BlackStar filesystem (bsfs) module.
-A copy of the license is provided with the project.
-Author: Matthias Baumgartner, 2022
-"""
 # imports
 import os
 import typing
 
 # bsfs imports
-from bsfs.schema import Schema
+from bsfs.query import ast, validate
+from bsfs import schema as bsc
 from bsfs.triple_store import TripleStoreBase
 from bsfs.utils import URI, typename
 
 # inner-module imports
+from . import ac
 from . import nodes as _nodes
+from . import resolve
 
 # exports
 __all__: typing.Sequence[str] = (
@@ -25,9 +23,7 @@ __all__: typing.Sequence[str] = (
 ## code ##
 
 class Graph():
-    """The Graph class is
-
-    The Graph class provides a convenient interface to query and access a graph.
+    """The Graph class provides a convenient interface to query and access a graph.
     Since it logically builds on the concept of graphs it is easier to
     navigate than raw triple stores. Naturally, it uses a triple store
     as *backend*. It also controls actions via access permissions to a *user*.
@@ -37,35 +33,40 @@ class Graph():
     # link to the triple storage backend.
     _backend: TripleStoreBase
 
-    # user uri.
-    _user: URI
+    # access controls.
+    _ac: ac.AccessControlBase
 
-    def __init__(self, backend: TripleStoreBase, user: URI):
+    def __init__(
+            self,
+            backend: TripleStoreBase,
+            access_control: ac.AccessControlBase,
+            ):
+        # store members
         self._backend = backend
-        self._user = user
+        self._ac = access_control
         # ensure Graph schema requirements
         self.migrate(self._backend.schema)
 
     def __hash__(self) -> int:
-        return hash((type(self), self._backend, self._user))
+        return hash((type(self), self._backend, self._ac))
 
     def __eq__(self, other) -> bool:
         return isinstance(other, type(self)) \
            and self._backend == other._backend \
-           and self._user == other._user
+           and self._ac == other._ac
 
     def __repr__(self) -> str:
-        return f'{typename(self)}(backend={repr(self._backend)}, user={self._user})'
+        return f'{typename(self)}({repr(self._backend)}, {self._ac})'
 
     def __str__(self) -> str:
-        return f'{typename(self)}({str(self._backend)}, {self._user})'
+        return f'{typename(self)}({str(self._backend)})'
 
     @property
-    def schema(self) -> Schema:
+    def schema(self) -> bsc.Schema:
         """Return the store's local schema."""
         return self._backend.schema
 
-    def migrate(self, schema: Schema, append: bool = True) -> 'Graph':
+    def migrate(self, schema: bsc.Schema, append: bool = True) -> 'Graph':
         """Migrate the current schema to a new *schema*.
 
         Appends to the current schema by default; control this via *append*.
@@ -73,14 +74,14 @@ class Graph():
 
         """
         # check args
-        if not isinstance(schema, Schema):
+        if not isinstance(schema, bsc.Schema):
             raise TypeError(schema)
         # append to current schema
         if append:
             schema = schema + self._backend.schema
         # add Graph schema requirements
         with open(os.path.join(os.path.dirname(__file__), 'schema.nt'), mode='rt', encoding='UTF-8') as ifile:
-            schema = schema + Schema.from_string(ifile.read())
+            schema = schema + bsc.from_string(ifile.read())
         # migrate schema in backend
         # FIXME: consult access controls!
         self._backend.schema = schema
@@ -95,19 +96,69 @@ class Graph():
         *node_type*) once some data is assigned to them.
 
         """
+        # get node type
         type_ = self.schema.node(node_type)
         # NOTE: Nodes constructor materializes guids.
-        return _nodes.Nodes(self._backend, self._user, type_, guids)
+        return _nodes.Nodes(self._backend, self._ac, type_, guids)
 
     def node(self, node_type: URI, guid: URI) -> _nodes.Nodes:
         """Return node *guid* of type *node_type* as a `bsfs.graph.Nodes` instance.
 
-        Note that the *guids* need not to exist (however, the *node_type* has
+        Note that the *guid* need not to exist (however, the *node_type* has
         to be part of the schema). An inexistent guid will be created (using
         *node_type*) once some data is assigned to them.
 
         """
+        return self.nodes(node_type, {guid})
+
+    def empty(self, node_type: URI) -> _nodes.Nodes:
+        """Return a `Nodes` instance with type *node_type* but no nodes."""
+        return self.nodes(node_type, set())
+
+    def get(
+            self,
+            node_type: URI,
+            query: typing.Optional[ast.filter.FilterExpression],
+            ) -> _nodes.Nodes:
+        """Return a `Nodes` instance over all nodes of type *node_type* that match the *query*."""
+        # return Nodes instance
+        type_ = self.schema.node(node_type)
+        return _nodes.Nodes(self._backend, self._ac, type_, self.__get(node_type, query))
+
+    def sorted(
+            self,
+            node_type: URI,
+            query: typing.Optional[ast.filter.FilterExpression],
+            # FIXME: sort ast
+            ) -> typing.Iterator[_nodes.Nodes]:
+        """Return a iterator over `Nodes` instances over all nodes of type *node_type* that match the *query*."""
+        # FIXME: Order should be a parameter
+        # return iterator over Nodes instances
+        type_ = self.schema.node(node_type)
+        for guid in self.__get(node_type, query):
+            yield _nodes.Nodes(self._backend, self._ac, type_, {guid})
+
+    def all(self, node_type: URI) -> _nodes.Nodes:
+        """Return all instances of type *node_type*."""
+        type_ = self.schema.node(node_type)
+        return _nodes.Nodes(self._backend, self._ac, type_, self.__get(node_type, None))
+
+    def __get(
+            self,
+            node_type: URI,
+            query: typing.Optional[ast.filter.FilterExpression],
+            ) -> typing.Iterator[URI]:
+        """Build and execute a get query."""
+        # get node type
         type_ = self.schema.node(node_type)
-        return _nodes.Nodes(self._backend, self._user, type_, {guid})
+        # resolve Nodes instances
+        query = resolve.Filter(self._backend.schema).resolve(type_, query)
+        # add access controls to query
+        query = self._ac.filter_read(type_, query)
+        # validate query
+        if query is not None:
+            validate.Filter(self._backend.schema).validate(type_, query)
+        # query the backend and return the (non-materialized) result
+        return self._backend.get(type_, query)
 
 ## EOF ##
diff --git a/bsfs/graph/nodes.py b/bsfs/graph/nodes.py
index c417a0e..47b0217 100644
--- a/bsfs/graph/nodes.py
+++ b/bsfs/graph/nodes.py
@@ -1,21 +1,20 @@
-"""
 
-Part of the BlackStar filesystem (bsfs) module.
-A copy of the license is provided with the project.
-Author: Matthias Baumgartner, 2022
-"""
 # imports
+from collections import abc
 import time
 import typing
 
 # bsfs imports
-from bsfs import schema as _schema
+from bsfs import schema as bsc
 from bsfs.namespace import ns
+from bsfs.query import ast, validate
 from bsfs.triple_store import TripleStoreBase
 from bsfs.utils import errors, URI, typename
 
 # inner-module imports
 from . import ac
+from . import result
+from . import walk
 
 # exports
 __all__: typing.Sequence[str] = (
@@ -26,18 +25,20 @@ __all__: typing.Sequence[str] = (
 ## code ##
 
 class Nodes():
-    """
+    """Container for graph nodes, provides operations on nodes.
+
+    NOTE: Should not be created directly but only via `bsfs.graph.Graph`.
     NOTE: guids may or may not exist. This is not verified as nodes are created on demand.
     """
 
     # triple store backend.
     _backend: TripleStoreBase
 
-    # user uri.
-    _user: URI
+    # access controls.
+    _ac: ac.AccessControlBase
 
     # node type.
-    _node_type: _schema.Node
+    _node_type: bsc.Node
 
     # guids of nodes. Can be empty.
     _guids: typing.Set[URI]
@@ -45,34 +46,35 @@ class Nodes():
     def __init__(
             self,
             backend: TripleStoreBase,
-            user: URI,
-            node_type: _schema.Node,
+            access_control: ac.AccessControlBase,
+            node_type: bsc.Node,
             guids: typing.Iterable[URI],
             ):
+        # set main members
         self._backend = backend
-        self._user = user
+        self._ac = access_control
         self._node_type = node_type
-        self._guids = set(guids)
-        self.__ac = ac.NullAC(self._backend, self._user)
+        # convert to URI since this is not guaranteed by Graph
+        self._guids = {URI(guid) for guid in guids}
 
     def __eq__(self, other: typing.Any) -> bool:
         return isinstance(other, Nodes) \
            and self._backend == other._backend \
-           and self._user == other._user \
+           and self._ac == other._ac \
            and self._node_type == other._node_type \
            and self._guids == other._guids
 
     def __hash__(self) -> int:
-        return hash((type(self), self._backend, self._user, self._node_type, tuple(sorted(self._guids))))
+        return hash((type(self), self._backend, self._ac, self._node_type, tuple(sorted(self._guids))))
 
     def __repr__(self) -> str:
-        return f'{typename(self)}({self._backend}, {self._user}, {self._node_type}, {self._guids})'
+        return f'{typename(self)}({self._backend}, {self._ac}, {self._node_type}, {self._guids})'
 
     def __str__(self) -> str:
         return f'{typename(self)}({self._node_type}, {self._guids})'
 
     @property
-    def node_type(self) -> _schema.Node:
+    def node_type(self) -> bsc.Node:
         """Return the node's type."""
         return self._node_type
 
@@ -81,9 +83,72 @@ class Nodes():
         """Return all node guids."""
         return iter(self._guids)
 
+    @property
+    def schema(self) -> bsc.Schema:
+        """Return the store's local schema."""
+        return self._backend.schema
+
+    def __add__(self, other: typing.Any) -> 'Nodes':
+        """Concatenate guids. Backend, AC, and node type must match."""
+        if not isinstance(other, type(self)):
+            return NotImplemented
+        if self._backend != other._backend:
+            raise ValueError(other)
+        if self._ac != other._ac:
+            raise ValueError(other)
+        if self.node_type != other.node_type:
+            raise ValueError(other)
+        return Nodes(self._backend, self._ac, self.node_type, self._guids | other._guids)
+
+    def __or__(self, other: typing.Any) -> 'Nodes':
+        """Concatenate guids. Backend, AC, and node type must match."""
+        return self.__add__(other)
+
+    def __sub__(self, other: typing.Any) -> 'Nodes':
+        """Subtract guids. Backend, AC, and node type must match."""
+        if not isinstance(other, type(self)):
+            return NotImplemented
+        if self._backend != other._backend:
+            raise ValueError(other)
+        if self._ac != other._ac:
+            raise ValueError(other)
+        if self.node_type != other.node_type:
+            raise ValueError(other)
+        return Nodes(self._backend, self._ac, self.node_type, self._guids - other._guids)
+
+    def __and__(self, other: typing.Any) -> 'Nodes':
+        """Intersect guids. Backend, AC, and node type must match."""
+        if not isinstance(other, type(self)):
+            return NotImplemented
+        if self._backend != other._backend:
+            raise ValueError(other)
+        if self._ac != other._ac:
+            raise ValueError(other)
+        if self.node_type != other.node_type:
+            raise ValueError(other)
+        return Nodes(self._backend, self._ac, self.node_type, self._guids & other._guids)
+
+    def __len__(self) -> int:
+        """Return the number of guids."""
+        return len(self._guids)
+
+    def __iter__(self) -> typing.Iterator['Nodes']:
+        """Iterate over individual guids. Returns `Nodes` instances."""
+        return iter(
+            Nodes(self._backend, self._ac, self.node_type, {guid})
+            for guid in self._guids
+            )
+
+    def __getattr__(self, name: str):
+        try:
+            return super().__getattr__(name) # type: ignore [misc] # parent has no getattr
+        except AttributeError:
+            pass
+        return walk.Walk(self, walk.Walk.step(self.schema, self.node_type, name))
+
     def set(
             self,
-            pred: URI, # FIXME: URI or _schema.Predicate?
+            pred: URI, # FIXME: URI or bsc.Predicate?
             value: typing.Any,
             ) -> 'Nodes':
         """Set predicate *pred* to *value*."""
@@ -91,7 +156,7 @@ class Nodes():
 
     def set_from_iterable(
             self,
-            predicate_values: typing.Iterable[typing.Tuple[URI, typing.Any]], # FIXME: URI or _schema.Predicate?
+            predicate_values: typing.Iterable[typing.Tuple[URI, typing.Any]], # FIXME: URI or bsc.Predicate?
             ) -> 'Nodes':
         """Set mutliple predicate-value pairs at once."""
         # TODO: Could group predicate_values by predicate to gain some efficiency
@@ -105,7 +170,7 @@ class Nodes():
             self._backend.commit()
 
         except (
-                errors.PermissionDeniedError, # tried to set a protected predicate (ns.bsm.t_created)
+                errors.PermissionDeniedError, # tried to set a protected predicate
                 errors.ConsistencyError, # node types are not in the schema or don't match the predicate
                 errors.InstanceError, # guids/values don't have the correct type
                 TypeError, # value is supposed to be a Nodes instance
@@ -120,6 +185,126 @@ class Nodes():
 
         return self
 
+    def get(
+            self,
+            *paths: typing.Union[URI, typing.Iterable[URI]],
+            view: typing.Union[typing.Type[list], typing.Type[dict]] = dict,
+            **view_kwargs,
+            ) -> typing.Any:
+        """Get values or nodes at *paths*.
+        Return an iterator (view=list) or a dict (view=dict) over the results.
+        """
+        # FIXME: user-provided Fetch query AST?
+        # check args
+        if len(paths) == 0:
+            raise AttributeError('expected at least one path, found none')
+        if view not in (dict, list):
+            raise ValueError(f'expected dict or list, found {view}')
+        # process paths: create fetch ast, build name mapping, and find unique paths
+        schema = self.schema
+        statements = set()
+        name2path = {}
+        unique_paths = set() # paths that result in a single (unique) value
+        normpath: typing.Tuple[URI, ...]
+        for idx, path in enumerate(paths):
+            # normalize path
+            if isinstance(path, str):
+                normpath = (URI(path), )
+            elif isinstance(path, abc.Iterable):
+                if not all(isinstance(step, str) for step in path):
+                    raise TypeError(path)
+                normpath = tuple(URI(step) for step in path)
+            else:
+                raise TypeError(path)
+            # check path's schema consistency
+            if not all(schema.has_predicate(pred) for pred in normpath):
+                raise errors.ConsistencyError(f'path is not fully covered by the schema: {path}')
+            # check path's uniqueness
+            if all(schema.predicate(pred).unique for pred in normpath):
+                unique_paths.add(path)
+            # fetch tail predicate
+            tail = schema.predicate(normpath[-1])
+            # determine tail ast node type
+            factory = ast.fetch.Node if isinstance(tail.range, bsc.Node) else ast.fetch.Value
+            # assign name
+            name = f'fetch{idx}'
+            name2path[name] = (path, tail)
+            # create tail ast node
+            curr: ast.fetch.FetchExpression = factory(tail.uri, name)
+            # walk towards front
+            hop: URI
+            for hop in normpath[-2::-1]:
+                curr = ast.fetch.Fetch(hop, curr)
+            # add to fetch query
+            statements.add(curr)
+        # aggregate fetch statements
+        if len(statements) == 1:
+            fetch = next(iter(statements))
+        else:
+            fetch = ast.fetch.All(*statements)
+        # add access controls to fetch
+        fetch = self._ac.fetch_read(self.node_type, fetch)
+
+        if len(self._guids) == 0:
+            # shortcut: no need to query; no triples
+            # FIXME: if the Fetch query can given by the user, we might want to check its validity
+            def triple_iter():
+                return []
+        else:
+            # compose filter ast
+            filter = ast.filter.IsIn(self.guids) # pylint: disable=redefined-builtin
+            # add access controls to filter
+            filter = self._ac.filter_read(self.node_type, filter) # type: ignore [assignment]
+
+            # validate queries
+            validate.Filter(self._backend.schema).validate(self.node_type, filter)
+            validate.Fetch(self._backend.schema).validate(self.node_type, fetch)
+
+            # process results, convert if need be
+            def triple_iter():
+                # query the backend
+                triples = self._backend.fetch(self.node_type, filter, fetch)
+                # process triples
+                for root, name, raw in triples:
+                    # get node
+                    node = Nodes(self._backend, self._ac, self.node_type, {root})
+                    # get path
+                    path, tail = name2path[name]
+                    # covert raw to value
+                    if isinstance(tail.range, bsc.Node):
+                        value = Nodes(self._backend, self._ac, tail.range, {raw})
+                    else:
+                        value = raw
+                    # emit triple
+                    yield node, path, value
+
+        # simplify by default
+        view_kwargs['node'] = view_kwargs.get('node', len(self._guids) != 1)
+        view_kwargs['path'] = view_kwargs.get('path', len(paths) != 1)
+        view_kwargs['value'] = view_kwargs.get('value', False)
+
+        # return results view
+        if view == list:
+            return result.to_list_view(
+                triple_iter(),
+                # aggregation args
+                **view_kwargs,
+                )
+
+        if view == dict:
+            return result.to_dict_view(
+                triple_iter(),
+                # context
+                len(self._guids) == 1,
+                len(paths) == 1,
+                unique_paths,
+                # aggregation args
+                **view_kwargs,
+                )
+
+        raise errors.UnreachableError() # view was already checked
+
+
     def __set(self, predicate: URI, value: typing.Any):
         """
         """
@@ -135,7 +320,7 @@ class Nodes():
         # FIXME: Needed? Could be integrated into other AC methods (by passing the predicate!)
         #        This could allow more fine-grained predicate control (e.g. based on ownership)
         #        rather than a global approach like this.
-        if self.__ac.is_protected_predicate(pred):
+        if self._ac.is_protected_predicate(pred):
             raise errors.PermissionDeniedError(pred)
 
         # set operation affects all nodes (if possible)
@@ -145,11 +330,11 @@ class Nodes():
         guids = set(self._ensure_nodes(node_type, guids))
 
         # check value
-        if isinstance(pred.range, _schema.Literal):
+        if isinstance(pred.range, bsc.Literal):
             # check write permissions on existing nodes
             # As long as the user has write permissions, we don't restrict
             # the creation or modification of literal values.
-            guids = set(self.__ac.write_literal(node_type, guids))
+            guids = set(self._ac.write_literal(node_type, guids))
 
             # insert literals
             # TODO: Support passing iterators as values for non-unique predicates
@@ -160,8 +345,9 @@ class Nodes():
                 [value],
                 )
 
-        elif isinstance(pred.range, _schema.Node):
+        elif isinstance(pred.range, bsc.Node):
             # check value type
+            # FIXME: value could be a set of Nodes
             if not isinstance(value, Nodes):
                 raise TypeError(value)
             # value's node_type must be a subclass of the predicate's range
@@ -172,14 +358,14 @@ class Nodes():
             # Link permissions cover adding and removing links on the source node.
             # Specifically, link permissions also allow to remove links to other
             # nodes if needed (e.g. for unique predicates).
-            guids = set(self.__ac.link_from_node(node_type, guids))
+            guids = set(self._ac.link_from_node(node_type, guids))
 
             # get link targets
             targets = set(value.guids)
             # ensure existence of value nodes; create nodes if need be
             targets = set(self._ensure_nodes(value.node_type, targets))
             # check link permissions on target nodes
-            targets = set(self.__ac.link_to_node(value.node_type, targets))
+            targets = set(self._ac.link_to_node(value.node_type, targets))
 
             # insert node links
             self._backend.set(
@@ -192,7 +378,7 @@ class Nodes():
         else:
             raise errors.UnreachableError()
 
-    def _ensure_nodes(self, node_type: _schema.Node, guids: typing.Iterable[URI]):
+    def _ensure_nodes(self, node_type: bsc.Node, guids: typing.Iterable[URI]):
         """
         """
         # check node existence
@@ -203,14 +389,14 @@ class Nodes():
         # create nodes if need be
         if len(missing) > 0:
             # check which missing nodes can be created
-            missing = set(self.__ac.createable(node_type, missing))
+            missing = set(self._ac.createable(node_type, missing))
             # create nodes
             self._backend.create(node_type, missing)
             # add bookkeeping triples
             self._backend.set(node_type, missing,
-                self._backend.schema.predicate(ns.bsm.t_created), [time.time()])
+                self._backend.schema.predicate(ns.bsn.t_created), [time.time()])
             # add permission triples
-            self.__ac.create(node_type, missing)
+            self._ac.create(node_type, missing)
         # return available nodes
         return existing | missing
 
diff --git a/bsfs/graph/resolve.py b/bsfs/graph/resolve.py
new file mode 100644
index 0000000..a58eb67
--- /dev/null
+++ b/bsfs/graph/resolve.py
@@ -0,0 +1,174 @@
+
+# imports
+import typing
+
+# bsfs imports
+from bsfs import schema as bsc
+from bsfs.query import ast
+from bsfs.utils import errors
+
+# inner-module imports
+from . import nodes
+
+# exports
+__all__: typing.Sequence[str] = (
+    'Filter',
+    )
+
+
+## code ##
+
+class Filter():
+    """Rewrites the query to replace `bsfs.graph.nodes.Nodes` instances with the respective URI.
+    Does only limited type checking and schema validation.
+    Use `bsfs.schema.validate.Filter` to do so.
+
+    Example:
+        input:  Any(ns.bse.tag, Is(Nodes(...)))
+        output: Any(ns.bse.tag, Or(Is(...), Is(...), ...)))
+
+    >>> tags = graph.node(ns.bsn.Tag, 'http://example.com/me/tag#1234')
+    >>> graph.get(ns.bsn.Entity, ast.filter.Any(ns.bse.tag, ast.filter.Is(tags)))
+
+    """
+
+    def __init__(self, schema):
+        self.schema = schema
+
+    def __call__(
+            self,
+            root_type: bsc.Node,
+            node: typing.Optional[ast.filter.FilterExpression],
+            ):
+        """Alias for `Resolve.resolve`."""
+        return self.resolve(root_type, node)
+
+    def resolve(
+            self,
+            root_type: bsc.Node,
+            node: typing.Optional[ast.filter.FilterExpression],
+            ):
+        """Resolve Nodes instances of a *node* query starting at *root_type*."""
+        if node is None:
+            return None
+        return self._parse_filter_expression(root_type, node)
+
+    def _parse_filter_expression(
+            self,
+            type_: bsc.Vertex,
+            node: ast.filter.FilterExpression,
+            ) -> ast.filter.FilterExpression:
+        """Route *node* to the handler of the respective FilterExpression subclass."""
+        if isinstance(node, ast.filter.Is):
+            return self._is(type_, node)
+        if isinstance(node, ast.filter.Not):
+            return self._not(type_, node)
+        if isinstance(node, ast.filter.Has):
+            return self._has(type_, node)
+        if isinstance(node, ast.filter.Any):
+            return self._any(type_, node)
+        if isinstance(node, ast.filter.All):
+            return self._all(type_, node)
+        if isinstance(node, ast.filter.And):
+            return self._and(type_, node)
+        if isinstance(node, ast.filter.Or):
+            return self._or(type_, node)
+        if isinstance(node, ast.filter.Distance):
+            return self._distance(type_, node)
+        if isinstance(node, (ast.filter.Equals, ast.filter.Substring, \
+                             ast.filter.StartsWith, ast.filter.EndsWith)):
+            return self._value(type_, node)
+        if isinstance(node, (ast.filter.LessThan, ast.filter.GreaterThan)):
+            return self._bounded(type_, node)
+        # invalid node
+        raise errors.BackendError(f'expected filter expression, found {node}')
+
+    def _parse_predicate_expression(self, node: ast.filter.PredicateExpression) -> bsc.Vertex:
+        """Route *node* to the handler of the respective PredicateExpression subclass."""
+        if isinstance(node, ast.filter.Predicate):
+            return self._predicate(node)
+        if isinstance(node, ast.filter.OneOf):
+            return self._one_of(node)
+        # invalid node
+        raise errors.BackendError(f'expected predicate expression, found {node}')
+
+    def _predicate(self, node: ast.filter.Predicate) -> bsc.Vertex:
+        if not self.schema.has_predicate(node.predicate):
+            raise errors.ConsistencyError(f'predicate {node.predicate} is not in the schema')
+        pred = self.schema.predicate(node.predicate)
+        dom, rng = pred.domain, pred.range
+        if node.reverse:
+            dom, rng = rng, dom
+        return rng
+
+    def _one_of(self, node: ast.filter.OneOf) -> bsc.Vertex:
+        # determine domain and range types
+        rng = None
+        for pred in node:
+            # parse child expression
+            subrng = self._parse_predicate_expression(pred)
+            # determine the next type
+            if rng is None or subrng > rng: # pick most generic range
+                rng = subrng
+            # check range consistency
+            if not subrng <= rng and not subrng >= rng:
+                raise errors.ConsistencyError(f'ranges {subrng} and {rng} are not related')
+        if not isinstance(rng, (bsc.Node, bsc.Literal)):
+            raise errors.BackendError(f'the range of node {node} is undefined')
+        return rng
+
+    def _any(self, type_: bsc.Vertex, node: ast.filter.Any) -> ast.filter.Any: # pylint: disable=unused-argument
+        next_type = self._parse_predicate_expression(node.predicate)
+        return ast.filter.Any(node.predicate, self._parse_filter_expression(next_type, node.expr))
+
+    def _all(self, type_: bsc.Vertex, node: ast.filter.All) -> ast.filter.All: # pylint: disable=unused-argument
+        next_type = self._parse_predicate_expression(node.predicate)
+        return ast.filter.All(node.predicate, self._parse_filter_expression(next_type, node.expr))
+
+    def _and(self, type_: bsc.Vertex, node: ast.filter.And) -> ast.filter.And:
+        return ast.filter.And({self._parse_filter_expression(type_, expr) for expr in node})
+
+    def _or(self, type_: bsc.Vertex, node: ast.filter.Or) -> ast.filter.Or:
+        return ast.filter.Or({self._parse_filter_expression(type_, expr) for expr in node})
+
+    def _not(self, type_: bsc.Vertex, node: ast.filter.Not) -> ast.filter.Not:
+        return ast.filter.Not(self._parse_filter_expression(type_, node.expr))
+
+    def _has(self, type_: bsc.Vertex, node: ast.filter.Has) -> ast.filter.Has: # pylint: disable=unused-argument
+        return node
+
+    def _distance(self, type_: bsc.Vertex, node: ast.filter.Distance): # pylint: disable=unused-argument
+        return node
+
+    def _value(self, type_: bsc.Vertex, node: ast.filter._Value) -> ast.filter._Value: # pylint: disable=unused-argument
+        return node
+
+    def _bounded(self, type_: bsc.Vertex, node: ast.filter._Bounded) -> ast.filter._Bounded: # pylint: disable=unused-argument
+        return node
+
+    def _is(self, type_: bsc.Vertex, node: ast.filter.Is) -> typing.Union[ast.filter.Or, ast.filter.Is]:
+        # check if action is needed
+        if not isinstance(node.value, nodes.Nodes):
+            return node
+        # check schema consistency
+        if node.value.node_type not in self.schema.nodes():
+            raise errors.ConsistencyError(f'node {node.value.node_type} is not in the schema')
+        # check type compatibility
+        if not isinstance(type_, bsc.Node):
+            raise errors.ConsistencyError(f'expected a node, found {type_}')
+        if not node.value.node_type <= type_:
+            raise errors.ConsistencyError(f'expected type {type_} or subtype thereof, found {node.value.node_type}')
+        # NOTE: We assume that the node type is checked when writing to the backend.
+        # Links to any of the guids can therefore only exist if the type matches.
+        # Hence, we don't add a type check/constrain here.
+        return ast.filter.Or(ast.filter.Is(guid) for guid in node.value.guids)
+        # optimized code, removing unnecessary ast.filter.Or
+        #guids = set(node.value.guids)
+        #if len(guids) == 0:
+        #    raise errors.BackendError(f'')
+        #if len(guids) == 1:
+        #    return ast.filter.Nodeid(next(iter(guids)))
+        #return ast.filter.Or(ast.filter.Is(guid) for guid in guids)
+
+
+## EOF ##
diff --git a/bsfs/graph/result.py b/bsfs/graph/result.py
new file mode 100644
index 0000000..0fcbb13
--- /dev/null
+++ b/bsfs/graph/result.py
@@ -0,0 +1,119 @@
+
+# imports
+from collections import defaultdict
+import typing
+
+# bsfs imports
+from bsfs.utils import URI
+
+# exports
+__all__: typing.Sequence[str] = (
+    'to_list_view',
+    'to_dict_view',
+    )
+
+
+## code ##
+
+# FIXME: node, path, value seem counter-intuitive:
+# node.get(..., node=True) removes the node part.
+# wouldn't it make more sense if node=True keeps the node part
+# and node=False drops it?
+
+def to_list_view(
+        triples,
+        # aggregators
+        node: bool,
+        path: bool,
+        value: bool, # pylint: disable=unused-argument
+        ):
+    """Return an iterator over results.
+
+    Dependent on the *node*, *path*, and *value* flags,
+    the respective component is omitted.
+
+    """
+    if not node and not path:
+        return iter(val for _, _, val in triples)
+    if not node:
+        return iter((pred, val) for _, pred, val in triples)
+    if not path:
+        return iter((subj, val) for subj, _, val in triples)
+    return iter((subj, pred, val) for subj, pred, val in triples)
+
+
+def to_dict_view(
+        triples,
+        # context
+        one_node: bool,
+        one_path: bool,
+        unique_paths: typing.Set[typing.Union[URI, typing.Iterable[URI]]],
+        # aggregators
+        node: bool,
+        path: bool,
+        value: bool,
+        default: typing.Optional[typing.Any] = None,
+        ) -> typing.Any:
+    """Return a dict of results.
+
+    Note that triples are materialized to create this view.
+
+    The returned structure depends on the *node*, *path*, and *value* flags.
+    If all flags are set to False, returns a dict(node -> dict(path -> set(values))).
+    Setting a flag to true omits or simplifies the respective component (if possible).
+
+    """
+    # NOTE: To create a dict, we need to materialize or make further assumptions
+    # (e.g., sorted in a specific order).
+
+    data: typing.Any # disable type checks on data since it's very flexibly typed.
+
+    # FIXME: type of data can be overwritten later on (if value)
+
+    if not node and not path:
+        data = set()
+    elif node ^ path:
+        data = defaultdict(set)
+    else:
+        data = defaultdict(lambda: defaultdict(set))
+
+    for subj, pred, val in triples:
+        unique = pred in unique_paths
+        if not node and not path:
+            if not value and unique and one_node and one_path:
+                return val
+            data.add(val)
+        elif not node:
+            # remove node from result, group by predicate
+            if not value and unique and one_node:
+                data[pred] = val
+            else:
+                data[pred].add(val)
+        elif not path:
+            # remove predicate from result, group by node
+            if not value and unique and one_path:
+                data[subj] = val
+            else:
+                data[subj].add(val)
+        else:
+            if not value and unique:
+                data[subj][pred] = val
+            else:
+                data[subj][pred].add(val)
+
+    # FIXME: Combine multiple Nodes instances into one?
+
+    # convert defaultdict to ordinary dict
+    # pylint: disable=too-many-boolean-expressions
+    if not node and not path and not value \
+       and len(unique_paths) > 0 and one_node and one_path \
+       and len(data) == 0:
+        return default
+    # pylint: enable=too-many-boolean-expressions
+    if not node and not path:
+        return data
+    if node ^ path:
+        return dict(data)
+    return {key: dict(val) for key, val in data.items()}
+
+## EOF ##
diff --git a/bsfs/graph/schema.nt b/bsfs/graph/schema.nt
index 8612681..37bba5e 100644
--- a/bsfs/graph/schema.nt
+++ b/bsfs/graph/schema.nt
@@ -4,15 +4,17 @@ prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
 prefix xsd: <http://www.w3.org/2001/XMLSchema#>
 
 # bsfs prefixes
-prefix bsfs: <http://bsfs.ai/schema/>
-prefix bsm: <http://bsfs.ai/schema/Meta#>
+prefix bsfs: <https://schema.bsfs.io/core/>
+prefix bsl: <https://schema.bsfs.io/core/Literal/>
+prefix bsn: <https://schema.bsfs.io/core/Node#>
 
 # literals
-xsd:integer rdfs:subClassOf bsfs:Literal .
+bsl:Number rdfs:subClassOf bsfs:Literal .
+xsd:float rdfs:subClassOf bsl:Number .
 
 # predicates
-bsm:t_created rdfs:subClassOf bsfs:Predicate ;
+bsn:t_created rdfs:subClassOf bsfs:Predicate ;
     rdfs:domain bsfs:Node ;
-    rdfs:range xsd:integer ;
+    rdfs:range xsd:float ;
     bsfs:unique "true"^^xsd:boolean .
 
diff --git a/bsfs/graph/walk.py b/bsfs/graph/walk.py
new file mode 100644
index 0000000..6415c9b
--- /dev/null
+++ b/bsfs/graph/walk.py
@@ -0,0 +1,115 @@
+
+# imports
+from collections import abc
+import typing
+
+# bsfs imports
+from bsfs import schema as bsc
+
+# inner-module imports
+# NOTE: circular import! OK as long as only used for type annotations.
+from . import nodes # pylint: disable=cyclic-import
+
+# exports
+__all__: typing.Sequence[str] = (
+    'Walk',
+    )
+
+
+## code ##
+
+class Walk(abc.Hashable, abc.Callable): # type: ignore [misc] # invalid base class (Callable)
+    """Syntactic sugar for `Nodes` to build and act on predicate paths via members."""
+
+    # Link to Nodes instance.
+    _root: 'nodes.Nodes'
+
+    # Current predicate path.
+    _path: typing.Tuple[bsc.Predicate, ...]
+
+    def __init__(
+            self,
+            root: 'nodes.Nodes',
+            path: typing.Sequence[bsc.Predicate],
+            ):
+        self._root = root
+        self._path = tuple(path)
+
+    @property
+    def tail(self):
+        """Return the node type at the end of the path."""
+        return self._path[-1].range
+
+
+    ## comparison
+
+    def __hash__(self) -> int:
+        """Return an integer hash that identifies the instance."""
+        return hash((type(self), self._root, self._path))
+
+    def __eq__(self, other) -> bool:
+        """Compare against *other* backend."""
+        return isinstance(other, type(self)) \
+           and self._root == other._root \
+           and self._path == other._path
+
+
+    ## representation
+
+    def __repr__(self) -> str:
+        """Return a formal string representation."""
+        path = ', '.join(pred.uri for pred in self._path)
+        return f'Walk({self._root.node_type.uri}, ({path}))'
+
+    def __str__(self) -> str:
+        """Return an informal string representation."""
+        path = ', '.join(pred.uri for pred in self._path)
+        return f'Walk(@{self._root.node_type.uri}: {path})'
+
+
+    ## walk
+
+    @staticmethod
+    def step(
+            schema: bsc.Schema,
+            node: bsc.Node,
+            name: str,
+            ) -> typing.Tuple[bsc.Predicate]:
+        """Get an predicate at *node* whose fragment matches *name*."""
+        predicates = tuple(
+            pred
+            for pred
+            in schema.predicates_at(node)
+            if pred.uri.get('fragment', None) == name
+            )
+        if len(predicates) == 0: # no fragment found for name
+            raise ValueError(f'no available predicate matches {name}') # FIXME: Custom exception
+        if len(predicates) > 1: # ambiguous name
+            raise ValueError(f'{name} matches multiple predicates') # FIXME: Custom exception
+        # append predicate to walk
+        return predicates # type: ignore [return-value] # size is one
+
+    def __getattr__(self, name: str) -> 'Walk':
+        """Alias for `Walk.step(name)`."""
+        try:
+            return super().__getattr__(name)
+        except AttributeError:
+            pass
+        # get predicate
+        pred = self.step(self._root.schema, self.tail, name)
+        # append predicate to walk
+        return Walk(self._root, self._path + pred)
+
+
+    ## get paths ##
+
+    def get(self, **kwargs) -> typing.Any:
+        """Alias for `Nodes.get(..)`."""
+        return self._root.get(tuple(pred.uri for pred in self._path), **kwargs)
+
+    def __call__(self, **kwargs) -> typing.Any: # pylint: disable=arguments-differ
+        """Alias for `Walk.get(...)`."""
+        return self.get(**kwargs)
+
+
+## EOF ##
diff --git a/bsfs/namespace/__init__.py b/bsfs/namespace/__init__.py
index 98d472f..76f39a2 100644
--- a/bsfs/namespace/__init__.py
+++ b/bsfs/namespace/__init__.py
@@ -1,19 +1,13 @@
-"""
 
-Part of the BlackStar filesystem (bsfs) module.
-A copy of the license is provided with the project.
-Author: Matthias Baumgartner, 2022
-"""
 # imports
 import typing
 
 # inner-module imports
 from . import predefined as ns
-from .namespace import ClosedNamespace, Namespace
+from .namespace import Namespace
 
 # exports
 __all__: typing.Sequence[str] = (
-    'ClosedNamespace',
     'Namespace',
     'ns',
     )
diff --git a/bsfs/namespace/namespace.py b/bsfs/namespace/namespace.py
index f652dcd..b388f53 100644
--- a/bsfs/namespace/namespace.py
+++ b/bsfs/namespace/namespace.py
@@ -1,104 +1,54 @@
-"""
 
-Part of the BlackStar filesystem (bsfs) module.
-A copy of the license is provided with the project.
-Author: Matthias Baumgartner, 2022
-"""
 # imports
 import typing
 
 # bsfs imports
-from bsfs.utils import URI, typename
+from bsfs.utils import URI
 
 # exports
 __all__: typing.Sequence[str] = (
-    'ClosedNamespace',
     'Namespace',
+    'FinalNamespace',
     )
 
 
 ## code ##
 
-class Namespace():
-    """A namespace consists of a common prefix that is used in a set of URIs.
 
-    Note that the prefix must include the separator between
-    path and fragment (typically a '#' or a '/').
-    """
-
-    # namespace prefix.
-    prefix: URI
-
-    # fragment separator.
-    fsep: str
-
-    # path separator.
-    psep: str
-
-    def __init__(self, prefix: URI, fsep: str = '#', psep: str = '/'):
-        # ensure prefix type
-        prefix = URI(prefix)
-        # truncate fragment separator
-        while prefix.endswith(fsep):
-            prefix = URI(prefix[:-1])
-        # truncate path separator
-        while prefix.endswith(psep):
-            prefix = URI(prefix[:-1])
-        # store members
-        self.prefix = prefix
-        self.fsep = fsep
-        self.psep = psep
-
-    def __eq__(self, other: typing.Any) -> bool:
-        return isinstance(other, type(self)) \
-           and self.prefix == other.prefix \
-           and self.fsep == other.fsep \
-           and self.psep == other.psep
+class Namespace(URI):
+    """The Namespace allows you to incrementally append path segments to an URI.
 
-    def __hash__(self) -> int:
-        return hash((type(self), self.prefix, self.fsep, self.psep))
+    Segments are separated by `Namespace.sep` ('/').
+    The `__call__` method signals that the URI is complete until the query part.
 
-    def __str__(self) -> str:
-        return f'{typename(self)}({self.prefix})'
-
-    def __repr__(self) -> str:
-        return f'{typename(self)}({self.prefix}, {self.fsep}, {self.psep})'
-
-    def __getattr__(self, fragment: str) -> URI:
-        """Return prefix + fragment."""
-        return URI(self.prefix + self.fsep + fragment)
-
-    def __getitem__(self, fragment: str) -> URI:
-        """Alias for getattr(self, fragment)."""
-        return self.__getattr__(fragment)
+    """
 
-    def __add__(self, value: typing.Any) -> 'Namespace':
-        """Concatenate another namespace to this one."""
-        if not isinstance(value, str):
-            return NotImplemented
-        return Namespace(self.prefix + self.psep + value, self.fsep, self.psep)
+    # path separator
+    sep: str = '/'
 
+    def __getattr__(self, query: str) -> 'Namespace':
+        """Append the *query* to the current value and return as Namespace."""
+        return Namespace(self + self.sep + query)
 
-class ClosedNamespace(Namespace):
-    """Namespace that covers a restricted set of URIs."""
+    def __call__(self, sep: str = '#') -> 'FinalNamespace':
+        """Finalize the namespace."""
+        return FinalNamespace(self, sep)
 
-    # set of permissible fragments.
-    fragments: typing.Set[str]
 
-    def __init__(self, prefix: URI, *args: str, fsep: str = '#', psep: str = '/'):
-        super().__init__(prefix, fsep, psep)
-        self.fragments = set(args)
+# FIXME: Integrate FinalNamespace into Namespace? Do we need to have both?
+class FinalNamespace(URI):
+    """The FinalNamespace allows you to append a fragment to an URI."""
 
-    def __eq__(self, other: typing.Any) -> bool:
-        return super().__eq__(other) and self.fragments == other.fragments
+    # fragment separator
+    sep: str
 
-    def __hash__(self) -> int:
-        return hash((type(self), self.prefix, tuple(sorted(self.fragments))))
+    def __new__(cls, value: str, sep: str = '#'):
+        inst = URI.__new__(cls, value)
+        inst.sep = sep
+        return inst
 
     def __getattr__(self, fragment: str) -> URI:
-        """Return prefix + fragment or raise a KeyError if the fragment is not part of this namespace."""
-        if fragment not in self.fragments:
-            raise KeyError(f'{fragment} is not a valid fragment of namespace {self.prefix}')
-        return super().__getattr__(fragment)
+        """Append the *fragment* to the current value and return as URI."""
+        return URI(self + self.sep + fragment)
 
 ## EOF ##
diff --git a/bsfs/namespace/predefined.py b/bsfs/namespace/predefined.py
index cd48a46..8b60d39 100644
--- a/bsfs/namespace/predefined.py
+++ b/bsfs/namespace/predefined.py
@@ -1,35 +1,29 @@
-"""
 
-Part of the BlackStar filesystem (bsfs) module.
-A copy of the license is provided with the project.
-Author: Matthias Baumgartner, 2022
-"""
 # imports
 import typing
 
-# bsfs imports
-from bsfs.utils import URI
-
 # inner-module imports
-from . import namespace
+from .namespace import Namespace, FinalNamespace
 
 # essential bsfs namespaces
-bsfs: namespace.Namespace = namespace.Namespace(URI('http://bsfs.ai/schema'), fsep='/')
-
+bsfs = Namespace('https://schema.bsfs.io/core')
 # additional bsfs namespaces
-bse: namespace.Namespace = namespace.Namespace(URI('http://bsfs.ai/schema/Entity'))
-bsm: namespace.Namespace = namespace.Namespace(URI('http://bsfs.ai/schema/Meta'))
+bsd = bsfs.distance()
+bsl = bsfs.Literal
+bsn = bsfs.Node()
 
 # generic namespaces
-rdf: namespace.Namespace = namespace.Namespace(URI('http://www.w3.org/1999/02/22-rdf-syntax-ns'))
-rdfs: namespace.Namespace = namespace.Namespace(URI('http://www.w3.org/2000/01/rdf-schema'))
-schema: namespace.Namespace = namespace.Namespace(URI('http://schema.org'), fsep='/')
-xsd: namespace.Namespace = namespace.Namespace(URI('http://www.w3.org/2001/XMLSchema'))
+rdf = FinalNamespace('http://www.w3.org/1999/02/22-rdf-syntax-ns')
+rdfs = FinalNamespace('http://www.w3.org/2000/01/rdf-schema')
+xsd = FinalNamespace('http://www.w3.org/2001/XMLSchema')
+schema = FinalNamespace('http://schema.org', sep='/')
 
+# exports
 __all__: typing.Sequence[str] = (
-    'bse',
+    'bsd',
     'bsfs',
-    'bsm',
+    'bsl',
+    'bsn',
     'rdf',
     'rdfs',
     'schema',
diff --git a/bsfs/query/__init__.py b/bsfs/query/__init__.py
new file mode 100644
index 0000000..58ff03a
--- /dev/null
+++ b/bsfs/query/__init__.py
@@ -0,0 +1,15 @@
+
+# imports
+import typing
+
+# inner-module imports
+from . import ast
+from . import validator as validate
+
+# exports
+__all__: typing.Sequence[str] = (
+    'ast',
+    'validate',
+    )
+
+## EOF ##
diff --git a/bsfs/query/ast/__init__.py b/bsfs/query/ast/__init__.py
new file mode 100644
index 0000000..bceaac0
--- /dev/null
+++ b/bsfs/query/ast/__init__.py
@@ -0,0 +1,23 @@
+"""Query AST components.
+
+The query AST consists of a Filter and a Fetch syntax trees.
+
+Classes beginning with an underscore (_) represent internal type hierarchies
+and should not be used for parsing. Note that the AST structures do not
+(and cannot) check semantic validity or consistency with a given schema.
+
+"""
+# imports
+import typing
+
+# inner-module imports
+from . import fetch
+from . import filter_ as filter # pylint: disable=redefined-builtin
+
+# exports
+__all__: typing.Sequence[str] = (
+    'fetch',
+    'filter',
+    )
+
+## EOF ##
diff --git a/bsfs/query/ast/fetch.py b/bsfs/query/ast/fetch.py
new file mode 100644
index 0000000..66d94e1
--- /dev/null
+++ b/bsfs/query/ast/fetch.py
@@ -0,0 +1,169 @@
+
+# imports
+from collections import abc
+import typing
+
+# bsfs imports
+from bsfs.utils import URI, typename, normalize_args
+
+# exports
+__all__ : typing.Sequence[str] = (
+    'All',
+    'Fetch',
+    'FetchExpression',
+    'Node',
+    'This',
+    'Value',
+    )
+
+
+## code ##
+
+class FetchExpression(abc.Hashable):
+    """Generic Fetch expression."""
+
+    def __repr__(self) -> str:
+        """Return the expressions's string representation."""
+        return f'{typename(self)}()'
+
+    def __hash__(self) -> int:
+        """Return the expression's integer representation."""
+        return hash(type(self))
+
+    def __eq__(self, other: typing.Any) -> bool:
+        """Return True if *self* and *other* are equivalent."""
+        return isinstance(other, type(self))
+
+
+class All(FetchExpression):
+    """Fetch all child expressions."""
+
+    # child expressions.
+    expr: typing.Set[FetchExpression]
+
+    def __init__(self, *expr):
+        # unpack child expressions
+        unfolded = set(normalize_args(*expr))
+        # check child expressions
+        if len(unfolded) == 0:
+            raise AttributeError('expected at least one expression, found none')
+        if not all(isinstance(itm, FetchExpression) for itm in unfolded):
+            raise TypeError(expr)
+        # initialize
+        super().__init__()
+        # assign members
+        self.expr = unfolded
+
+    def __iter__(self) -> typing.Iterator[FetchExpression]:
+        return iter(self.expr)
+
+    def __len__(self) -> int:
+        return len(self.expr)
+
+    def __repr__(self) -> str:
+        return f'{typename(self)}({self.expr})'
+
+    def __hash__(self) -> int:
+        return hash((super().__hash__(), tuple(sorted(self.expr, key=repr))))
+
+    def __eq__(self, other: typing.Any) -> bool:
+        return super().__eq__(other) and self.expr == other.expr
+
+
+class _Branch(FetchExpression):
+    """Branch along a predicate."""
+
+    # FIXME: Use a Predicate (like in ast.filter) so that we can also reverse them!
+
+    # predicate to follow.
+    predicate: URI
+
+    def __init__(self, predicate: URI):
+        if not isinstance(predicate, URI):
+            raise TypeError(predicate)
+        self.predicate = predicate
+
+    def __repr__(self) -> str:
+        return f'{typename(self)}({self.predicate})'
+
+    def __hash__(self) -> int:
+        return hash((super().__hash__(), self.predicate))
+
+    def __eq__(self, other: typing.Any) -> bool:
+        return super().__eq__(other) and self.predicate == other.predicate
+
+
+class Fetch(_Branch):
+    """Follow a predicate before evaluating a child epxression."""
+
+    # child expression.
+    expr: FetchExpression
+
+    def __init__(self, predicate: URI, expr: FetchExpression):
+        # check child expressions
+        if not isinstance(expr, FetchExpression):
+            raise TypeError(expr)
+        # initialize
+        super().__init__(predicate)
+        # assign members
+        self.expr = expr
+
+    def __repr__(self) -> str:
+        return f'{typename(self)}({self.predicate}, {self.expr})'
+
+    def __hash__(self) -> int:
+        return hash((super().__hash__(), self.expr))
+
+    def __eq__(self, other: typing.Any) -> bool:
+        return super().__eq__(other) and self.expr == other.expr
+
+
+class _Named(_Branch):
+    """Fetch a (named) symbol at a predicate."""
+
+    # symbol name.
+    name: str
+
+    def __init__(self, predicate: URI, name: str):
+        super().__init__(predicate)
+        self.name = str(name)
+
+    def __repr__(self) -> str:
+        return f'{typename(self)}({self.predicate}, {self.name})'
+
+    def __hash__(self) -> int:
+        return hash((super().__hash__(), self.name))
+
+    def __eq__(self, other: typing.Any) -> bool:
+        return super().__eq__(other) and self.name == other.name
+
+
+class Node(_Named): # pylint: disable=too-few-public-methods
+    """Fetch a Node at a predicate."""
+    # FIXME: Is this actually needed?
+
+
+class Value(_Named): # pylint: disable=too-few-public-methods
+    """Fetch a Literal at a predicate."""
+
+
+class This(FetchExpression):
+    """Fetch the current Node."""
+
+    # symbol name.
+    name: str
+
+    def __init__(self, name: str):
+        super().__init__()
+        self.name = str(name)
+
+    def __repr__(self) -> str:
+        return f'{typename(self)}({self.name})'
+
+    def __hash__(self) -> int:
+        return hash((super().__hash__(), self.name))
+
+    def __eq__(self, other: typing.Any) -> bool:
+        return super().__eq__(other) and self.name == other.name
+
+## EOF ##
diff --git a/bsfs/query/ast/filter_.py b/bsfs/query/ast/filter_.py
new file mode 100644
index 0000000..610fdb4
--- /dev/null
+++ b/bsfs/query/ast/filter_.py
@@ -0,0 +1,516 @@
+"""Filter AST.
+
+Note that it is easily possible to construct an AST that is inconsistent with
+a given schema. Furthermore, it is possible to construct a semantically invalid
+AST which that cannot be parsed correctly or includes contradicting statements.
+The AST nodes do not (and cannot) check such issues.
+
+For example, consider the following AST:
+
+>>> Any(ns.bse.collection,
+...     And(
+...         Equals('hello'),
+...         Is('hello world'),
+...         Any(ns.bse.tag, Equals('world')),
+...         Any(ns.bst.label, Equals('world')),
+...         All(ns.bst.label, Not(Equals('world'))),
+...     )
+... )
+
+This AST has multiple issues that are not verified upon its creation:
+* A condition on a non-literal.
+* A Filter on a literal.
+* Conditions exclude each other
+* The predicate along the branch have incompatible domains and ranges.
+
+"""
+# imports
+from collections import abc
+import typing
+
+# bsfs imports
+from bsfs.utils import URI, typename, normalize_args
+
+# exports
+__all__ : typing.Sequence[str] = (
+    # base classes
+    'FilterExpression',
+    'PredicateExpression',
+    # predicate expressions
+    'OneOf',
+    'Predicate',
+    # branching
+    'All',
+    'Any',
+    # aggregators
+    'And',
+    'Or',
+    # value matchers
+    'Equals',
+    'Substring',
+    'EndsWith',
+    'StartsWith',
+    # range matchers
+    'GreaterThan',
+    'LessThan',
+    # misc
+    'Has',
+    'Is',
+    'Not',
+    )
+
+
+## code ##
+
+# pylint: disable=too-few-public-methods # Many expressions use mostly magic methods
+
+class _Expression(abc.Hashable):
+    def __repr__(self) -> str:
+        """Return the expressions's string representation."""
+        return f'{typename(self)}()'
+
+    def __hash__(self) -> int:
+        """Return the expression's integer representation."""
+        return hash(type(self))
+
+    def __eq__(self, other: typing.Any) -> bool:
+        """Return True if *self* and *other* are equivalent."""
+        return isinstance(other, type(self))
+
+
+class FilterExpression(_Expression):
+    """Generic Filter expression."""
+
+
+class PredicateExpression(_Expression):
+    """Generic Predicate expression."""
+
+
+class _Branch(FilterExpression):
+    """Branch the filter along a predicate."""
+
+    # predicate to follow.
+    predicate: PredicateExpression
+
+    # child expression to evaluate.
+    expr: FilterExpression
+
+    def __init__(
+            self,
+            predicate: typing.Union[PredicateExpression, URI],
+            expr: FilterExpression,
+            ):
+        # process predicate argument
+        if isinstance(predicate, URI):
+            predicate = Predicate(predicate)
+        elif not isinstance(predicate, PredicateExpression):
+            raise TypeError(predicate)
+        # process expression argument
+        if not isinstance(expr, FilterExpression):
+            raise TypeError(expr)
+        # assign members
+        self.predicate = predicate
+        self.expr = expr
+
+    def __repr__(self) -> str:
+        return f'{typename(self)}({self.predicate}, {self.expr})'
+
+    def __hash__(self) -> int:
+        return hash((super().__hash__(), self.predicate, self.expr))
+
+    def __eq__(self, other) -> bool:
+        return super().__eq__(other) \
+           and self.predicate == other.predicate \
+           and self.expr == other.expr
+
+class Any(_Branch):
+    """Any (and at least one) triple matches."""
+
+
+class All(_Branch):
+    """All (and at least one) triples match."""
+
+
+class _Agg(FilterExpression, abc.Collection):
+    """Combine multiple expressions."""
+
+    # child expressions
+    expr: typing.Set[FilterExpression]
+
+    def __init__(
+            self,
+            *expr: typing.Union[FilterExpression,
+                                typing.Iterable[FilterExpression],
+                                typing.Iterator[FilterExpression]]
+            ):
+        # unfold arguments
+        unfolded = set(normalize_args(*expr))
+        # check type
+        if not all(isinstance(e, FilterExpression) for e in unfolded):
+            raise TypeError(expr)
+        # FIXME: Require at least one child expression?
+        # assign member
+        self.expr = unfolded
+
+    def __contains__(self, expr: typing.Any) -> bool:
+        """Return True if *expr* is among the child expressions."""
+        return expr in self.expr
+
+    def __iter__(self) -> typing.Iterator[FilterExpression]:
+        """Iterator over child expressions."""
+        return iter(self.expr)
+
+    def __len__(self) -> int:
+        """Number of child expressions."""
+        return len(self.expr)
+
+    def __repr__(self) -> str:
+        return f'{typename(self)}({self.expr})'
+
+    def __hash__(self) -> int:
+        return hash((super().__hash__(), tuple(sorted(self.expr, key=repr))))
+
+    def __eq__(self, other) -> bool:
+        return super().__eq__(other) and self.expr == other.expr
+
+
+class And(_Agg):
+    """All conditions match."""
+
+
+class Or(_Agg):
+    """At least one condition matches."""
+
+
+class Not(FilterExpression):
+    """Invert a statement."""
+
+    # child expression
+    expr: FilterExpression
+
+    def __init__(self, expr: FilterExpression):
+        # check argument
+        if not isinstance(expr, FilterExpression):
+            raise TypeError(expr)
+        # assign member
+        self.expr = expr
+
+    def __repr__(self) -> str:
+        return f'{typename(self)}({self.expr})'
+
+    def __hash__(self) -> int:
+        return hash((super().__hash__(), self.expr))
+
+    def __eq__(self, other: typing.Any) -> bool:
+        return super().__eq__(other) and self.expr == other.expr
+
+
+class Has(FilterExpression):
+    """Has predicate N times"""
+
+    # predicate to follow.
+    predicate: PredicateExpression
+
+    # target count
+    count: FilterExpression
+
+    def __init__(
+            self,
+            predicate: typing.Union[PredicateExpression, URI],
+            count: typing.Optional[typing.Union[FilterExpression, int]] = None,
+            ):
+        # check predicate
+        if isinstance(predicate, URI):
+            predicate = Predicate(predicate)
+        elif not isinstance(predicate, PredicateExpression):
+            raise TypeError(predicate)
+        # check count
+        if count is None:
+            count = GreaterThan(1, strict=False)
+        elif isinstance(count, int):
+            count = Equals(count)
+        elif not isinstance(count, FilterExpression):
+            raise TypeError(count)
+        # assign members
+        self.predicate = predicate
+        self.count = count
+
+    def __repr__(self) -> str:
+        return f'{typename(self)}({self.predicate}, {self.count})'
+
+    def __hash__(self) -> int:
+        return hash((super().__hash__(), self.predicate, self.count))
+
+    def __eq__(self, other) -> bool:
+        return super().__eq__(other) \
+           and self.predicate == other.predicate \
+           and self.count == other.count
+
+
+class _Value(FilterExpression):
+    """Matches some value."""
+
+    # target value.
+    value: typing.Any
+
+    def __init__(self, value: typing.Any):
+        self.value = value
+
+    def __repr__(self) -> str:
+        return f'{typename(self)}({self.value})'
+
+    def __hash__(self) -> int:
+        return hash((super().__hash__(), self.value))
+
+    def __eq__(self, other) -> bool:
+        return super().__eq__(other) and self.value == other.value
+
+
+class Is(_Value):
+    """Match the URI of a node."""
+
+
+class Equals(_Value):
+    """Value matches exactly.
+    NOTE: Value must correspond to literal type.
+    """
+
+
+class Substring(_Value):
+    """Value matches a substring
+    NOTE: value must be a string.
+    """
+
+
+class StartsWith(_Value):
+    """Value begins with a given string."""
+
+
+class EndsWith(_Value):
+    """Value ends with a given string."""
+
+
+class Distance(FilterExpression):
+    """Distance to a reference is (strictly) below a threshold. Assumes a Feature literal."""
+
+    # FIXME:
+    # (a) pass a node/predicate as anchor instead of a value.
+    #     Then we don't need to materialize the reference.
+    # (b) pass a FilterExpression (_Bounded) instead of a threshold.
+    #     Then, we could also query values greater than a threshold.
+
+    # reference value.
+    reference: typing.Any
+
+    # distance threshold.
+    threshold: float
+
+    # closed (True) or open (False) bound.
+    strict: bool
+
+    def __init__(
+            self,
+            reference: typing.Any,
+            threshold: float,
+            strict: bool = False,
+            ):
+        self.reference = reference
+        self.threshold = float(threshold)
+        self.strict = bool(strict)
+
+    def __repr__(self) -> str:
+        return f'{typename(self)}({self.reference}, {self.threshold}, {self.strict})'
+
+    def __hash__(self) -> int:
+        return hash((super().__hash__(), tuple(self.reference), self.threshold, self.strict))
+
+    def __eq__(self, other) -> bool:
+        return super().__eq__(other) \
+           and self.reference == other.reference \
+           and self.threshold == other.threshold \
+           and self.strict == other.strict
+
+
+class _Bounded(FilterExpression):
+    """Value is bounded by a threshold. Assumes a Number literal."""
+
+    # bound.
+    threshold: float
+
+    # closed (True) or open (False) bound.
+    strict: bool
+
+    def __init__(
+            self,
+            threshold: float,
+            strict: bool = True,
+            ):
+        self.threshold = float(threshold)
+        self.strict = bool(strict)
+
+    def __repr__(self) -> str:
+        return f'{typename(self)}({self.threshold}, {self.strict})'
+
+    def __hash__(self) -> int:
+        return hash((super().__hash__(), self.threshold, self.strict))
+
+    def __eq__(self, other) -> bool:
+        return super().__eq__(other) \
+           and self.threshold == other.threshold \
+           and self.strict == other.strict
+
+
+
+class LessThan(_Bounded):
+    """Value is (strictly) smaller than threshold. Assumes a Number literal."""
+
+
+class GreaterThan(_Bounded):
+    """Value is (strictly) larger than threshold. Assumes a Number literal."""
+
+
+class Predicate(PredicateExpression):
+    """A single predicate."""
+
+    # predicate URI
+    predicate: URI
+
+    # reverse the predicate's direction
+    reverse: bool
+
+    def __init__(
+            self,
+            predicate: URI,
+            reverse: typing.Optional[bool] = False,
+            ):
+        # check arguments
+        if not isinstance(predicate, URI):
+            raise TypeError(predicate)
+        # assign members
+        self.predicate = predicate
+        self.reverse = bool(reverse)
+
+    def __repr__(self) -> str:
+        return f'{typename(self)}({self.predicate}, {self.reverse})'
+
+    def __hash__(self) -> int:
+        return hash((super().__hash__(), self.predicate, self.reverse))
+
+    def __eq__(self, other) -> bool:
+        return super().__eq__(other) \
+           and self.predicate == other.predicate \
+           and self.reverse == other.reverse
+
+
+class OneOf(PredicateExpression, abc.Collection):
+    """A set of predicate alternatives.
+
+    The predicates' domains must be ascendants or descendants of each other.
+    The overall domain is the most specific one.
+
+    The predicate's domains must be ascendants or descendants of each other.
+    The overall range is the most generic one.
+    """
+
+    # predicate alternatives
+    expr: typing.Set[PredicateExpression]
+
+    def __init__(self, *expr: typing.Union[PredicateExpression, URI]):
+        # unfold arguments
+        unfolded = set(normalize_args(*expr)) # type: ignore [arg-type] # this is getting too complex...
+        # check arguments
+        if len(unfolded) == 0:
+            raise AttributeError('expected at least one expression, found none')
+        # ensure PredicateExpression
+        unfolded = {Predicate(e) if isinstance(e, URI) else e for e in unfolded}
+        # check type
+        if not all(isinstance(e, PredicateExpression) for e in unfolded):
+            raise TypeError(expr)
+        # assign member
+        self.expr = unfolded
+
+    def __contains__(self, expr: typing.Any) -> bool:
+        """Return True if *expr* is among the child expressions."""
+        return expr in self.expr
+
+    def __iter__(self) -> typing.Iterator[PredicateExpression]:
+        """Iterator over child expressions."""
+        return iter(self.expr)
+
+    def __len__(self) -> int:
+        """Number of child expressions."""
+        return len(self.expr)
+
+    def __repr__(self) -> str:
+        return f'{typename(self)}({self.expr})'
+
+    def __hash__(self) -> int:
+        return hash((super().__hash__(), tuple(sorted(self.expr, key=repr))))
+
+    def __eq__(self, other) -> bool:
+        return super().__eq__(other) and self.expr == other.expr
+
+
+# Helpers
+# invalid-name is disabled since they explicitly mimic an expression
+
+def IsIn(*values) -> FilterExpression: # pylint: disable=invalid-name
+    """Match any of the given URIs."""
+    args = normalize_args(*values)
+    if len(args) == 0:
+        raise AttributeError('expected at least one value, found none')
+    if len(args) == 1:
+        return Is(args[0])
+    return Or(Is(value) for value in args)
+
+def IsNotIn(*values) -> FilterExpression: # pylint: disable=invalid-name
+    """Match none of the given URIs."""
+    return Not(IsIn(*values))
+
+
+def Between( # pylint: disable=invalid-name
+        lo: float = float('-inf'),
+        hi: float = float('inf'),
+        lo_strict: bool = True,
+        hi_strict: bool = True,
+        ) -> FilterExpression :
+    """Match numerical values between *lo* and *hi*. Include bounds if strict is False."""
+    if abs(lo) == hi == float('inf'):
+        raise ValueError('range cannot be INF on both sides')
+    if lo > hi:
+        raise ValueError(f'lower bound ({lo}) cannot be less than upper bound ({hi})')
+    if lo == hi and not lo_strict and not hi_strict:
+        return Equals(lo)
+    if lo == hi: # either bound is strict
+        raise ValueError('bounds cannot be equal when either is strict')
+    if lo != float('-inf') and hi != float('inf'):
+        return And(GreaterThan(lo, lo_strict), LessThan(hi, hi_strict))
+    if lo != float('-inf'):
+        return GreaterThan(lo, lo_strict)
+    # hi != float('inf'):
+    return LessThan(hi, hi_strict)
+
+
+def Includes(*values, approx: bool = False) -> FilterExpression: # pylint: disable=invalid-name
+    """Match any of the given *values*. Uses `Substring` if *approx* is set."""
+    args = normalize_args(*values)
+    cls = Substring if approx else Equals
+    if len(args) == 0:
+        raise AttributeError('expected at least one value, found none')
+    if len(args) == 1:
+        return cls(args[0])
+    return Or(cls(v) for v in args)
+
+
+def Excludes(*values, approx: bool = False) -> FilterExpression: # pylint: disable=invalid-name
+    """Match none of the given *values*. Uses `Substring` if *approx* is set."""
+    args = normalize_args(*values)
+    cls = Substring if approx else Equals
+    if len(args) == 0:
+        raise AttributeError('expected at least one value, found none')
+    if len(args) == 1:
+        return Not(cls(args[0]))
+    return Not(Or(cls(v) for v in args))
+
+
+## EOF ##
diff --git a/bsfs/query/matcher.py b/bsfs/query/matcher.py
new file mode 100644
index 0000000..17c9c8e
--- /dev/null
+++ b/bsfs/query/matcher.py
@@ -0,0 +1,361 @@
+
+# imports
+from collections import defaultdict
+from itertools import product
+from time import time
+import random
+import threading
+import typing
+
+# external imports
+from hopcroftkarp import HopcroftKarp
+
+# bsfs imports
+from bsfs.utils import errors, typename
+
+# inner-module imports
+from . import ast
+
+# exports
+__all__ : typing.Sequence[str] = (
+    'Filter',
+    )
+
+
+## code ##
+
+class Any(ast.filter.FilterExpression, ast.filter.PredicateExpression):
+    """Match any ast class.
+
+    Note that Any instances are unique, i.e. they do not compare, and
+    can hence be repeated in a set:
+    >>> Any() == Any()
+    False
+    >>> len({Any(), Any(), Any(), Any()})
+    4
+
+    """
+
+    # unique instance id
+    _uid: typing.Tuple[int, int, float, float]
+
+    def __init__(self):
+        self._uid = (
+            id(self),
+            id(threading.current_thread()),
+            time(),
+            random.random(),
+            )
+
+    def __eq__(self, other: typing.Any):
+        return super().__eq__(other) and self._uid == other._uid
+
+    def __hash__(self):
+        return hash((super().__hash__(), self._uid))
+
+
+class Rest(ast.filter.FilterExpression, ast.filter.PredicateExpression):
+    """Match the leftovers in a set of items to be compared.
+
+    Rest can be used in junction with aggregating expressions such as ast.filter.And,
+    ast.filter.Or, ast.filter.OneOf. It controls childs expressions that were not yet
+    consumed by other matching rules. Rest may match to only a specific expression.
+    The expresssion defaults to Any().
+
+    For example, the following to ast structures would match since Rest
+    allows an arbitrary repetition of ast.filter.Equals statements.
+
+    >>> And(Equals('hello'), Equals('world'), Equals('foobar'))
+    >>> And(Equals('world'), Rest(Partial(Equals)))
+
+    """
+
+    # child expression for the Rest.
+    expr: typing.Union[ast.filter.FilterExpression, ast.filter.PredicateExpression]
+
+    def __init__(
+            self,
+            expr: typing.Optional[typing.Union[ast.filter.FilterExpression, ast.filter.PredicateExpression]] = None,
+            ):
+        if expr is None:
+            expr = Any()
+        self.expr = expr
+
+    def __repr__(self) -> str:
+        return f'{typename(self)}({self.expr})'
+
+    def __hash__(self) -> int:
+        return hash((super().__hash__(), self.expr))
+
+    def __eq__(self, other: typing.Any) -> bool:
+        return super().__eq__(other) and self.expr == other.expr
+
+
+class Partial(ast.filter.FilterExpression, ast.filter.PredicateExpression):
+    """Match a partially defined ast expression.
+
+    Literal values might be irrelevant or unknown when comparing two ast
+    structures. Partial allows to constrain the matcher to a certain
+    ast class, while leaving some of its members unspecified.
+
+    Pass the class (not instance) and its members as keyword arguments
+    to Partial. Note that the arguments are not validated.
+
+    For example, the following instance matches any ast.filter.Equals,
+    irrespective of its value:
+
+    >>> Partial(ast.filter.Equals)
+
+    Likewise, the following instance matches any ast.filter.LessThan
+    that has a strict bounds, but makes no claim about the threshold:
+
+    >>> Partial(ast.filter.LessThan, strict=False)
+
+    """
+
+    # target node type.
+    node: typing.Type
+
+    # node construction args.
+    kwargs: typing.Dict[str, typing.Any]
+
+    def __init__(
+            self,
+            node: typing.Type,
+            **kwargs,
+            ):
+        self.node = node
+        self.kwargs = kwargs
+
+    def __repr__(self) -> str:
+        return f'{typename(self)}({self.node.__name__}, {self.kwargs})'
+
+    def __hash__(self) -> int:
+        kwargs = tuple((key, self.kwargs[key]) for key in sorted(self.kwargs))
+        return hash((super().__hash__(), self.node, kwargs))
+
+    def __eq__(self, other: typing.Any) -> bool:
+        return super().__eq__(other) \
+           and self.node == other.node \
+           and self.kwargs == other.kwargs
+
+    def match(
+            self,
+            name: str,
+            value: typing.Any,
+            ) -> bool:
+        """Return True if *name* is unspecified or matches *value*."""
+        return name not in self.kwargs or self.kwargs[name] == value
+
+
+T_ITEM_TYPE = typing.TypeVar('T_ITEM_TYPE') # pylint: disable=invalid-name
+
+def _set_matcher(
+        query: typing.Collection[T_ITEM_TYPE],
+        reference: typing.Collection[T_ITEM_TYPE],
+        cmp: typing.Callable[[T_ITEM_TYPE, T_ITEM_TYPE], bool],
+        ) -> bool:
+    """Compare two sets of child expressions.
+
+    This check has a best-case complexity of O(|N|**2) and worst-case
+    complexity of O(|N|**3), with N the number of child expressions.
+    """
+    # get reference items
+    r_items = list(reference)
+    # deal with Rest
+    r_rest = {itm for itm in r_items if isinstance(itm, Rest)}
+    if len(r_rest) > 1:
+        raise errors.BackendError(f'there must be at most one Rest instance per set, found {len(r_rest)}')
+    if len(r_rest) == 1:
+        # replace Rest by filling the reference up with rest's expression
+        # NOTE: convert r_items to list so that items can be repeated
+        expr = next(iter(r_rest)).expr # type: ignore [attr-defined]
+        r_items = [itm for itm in r_items if not isinstance(itm, Rest)]
+        r_items += [expr for _ in range(len(query) - len(r_items))] # type: ignore [misc]
+    # sanity check: cannot match if the item sizes differ:
+    # either a reference item is unmatched (len(r_items) > len(query))
+    # or a query item is unmatched (len(r_items) < len(query))
+    if len(query) != len(r_items):
+        return False
+
+    # To have a positive match between the query and the reference,
+    # each query expr has to match any reference expr.
+    # However, each reference expr can only be "consumed" once even
+    # if it matches multiple query exprs (e.g., the Any expression matches
+    # every query expr).
+    # This is a bipartide matching problem (Hall's marriage problem)
+    # and the Hopcroft-Karp-Karzanov algorithm finds a maximum
+    # matching. While there might be multiple maximum matchings,
+    # we only need to know whether (at least) one complete matching
+    # exists. The hopcroftkarp module provides this functionality.
+    # The HKK algorithm has worst-case complexity of O(|N|**2 * sqrt(|N|))
+    # and we also need to compare expressions pairwise, hence O(|N|**2).
+    num_items = len(r_items)
+    graph = defaultdict(set)
+    # build the bipartide graph as {lhs: {rhs}, ...}
+    # lhs and rhs must be disjoint identifiers.
+    for (ridx, ref), (nidx, node) in product(enumerate(r_items), enumerate(query)):
+        # add edges for equal expressions
+        if cmp(node, ref):
+            graph[ridx].add(num_items + nidx)
+
+    # maximum_matching returns the matches for all nodes in the graph
+    # ({ref_itm: node_itm}), hence a complete matching's size is
+    # the number of reference's child expressions.
+    return len(HopcroftKarp(graph).maximum_matching(keys_only=True)) == num_items
+
+
+class Filter():
+    """Compare a bsfs.query.ast.filter` query's structure to a reference ast.
+
+    The reference ast may include `Rest`, `Partial`, or `Any` to account for irrelevant
+    or unknown ast pieces.
+
+    This is only a structural comparison, not a semantic one. For example, the
+    two following queries are semantically identical, but structurally different,
+    and would therefore not match:
+
+    >>> ast.filter.OneOf(ast.filter.Predicate(ns.bse.name))
+    >>> ast.filter.Predicate(ns.bse.name)
+
+    """
+
+    def __call__(self, query: ast.filter.FilterExpression, reference: ast.filter.FilterExpression) -> bool:
+        """Compare a *query* to a *reference* ast structure.
+        Return True if both are structurally equivalent.
+        """
+        if not isinstance(query, ast.filter.FilterExpression):
+            raise errors.BackendError(f'expected filter expression, found {query}')
+        if not isinstance(reference, ast.filter.FilterExpression):
+            raise errors.BackendError(f'expected filter expression, found {reference}')
+        return self._parse_filter_expression(query, reference)
+
+    def _parse_filter_expression(
+            self,
+            node: ast.filter.FilterExpression,
+            reference: ast.filter.FilterExpression,
+            ) -> bool:
+        """Route *node* to the handler of the respective FilterExpression subclass."""
+        # generic checks: reference type must be Any or match node type
+        if isinstance(reference, Any):
+            return True
+        # node-specific checks
+        if isinstance(node, ast.filter.Not):
+            return self._not(node, reference)
+        if isinstance(node, ast.filter.Has):
+            return self._has(node, reference)
+        if isinstance(node, ast.filter.Distance):
+            return self._distance(node, reference)
+        if isinstance(node, (ast.filter.Any, ast.filter.All)):
+            return self._branch(node, reference)
+        if isinstance(node, (ast.filter.And, ast.filter.Or)):
+            return self._agg(node, reference)
+        if isinstance(node, (ast.filter.Is, ast.filter.Equals, ast.filter.Substring,
+                             ast.filter.StartsWith, ast.filter.EndsWith)):
+            return self._value(node, reference)
+        if isinstance(node, (ast.filter.LessThan, ast.filter.GreaterThan)):
+            return self._bounded(node, reference)
+        # invalid node
+        raise errors.BackendError(f'expected filter expression, found {node}')
+
+    def _parse_predicate_expression(
+            self,
+            node: ast.filter.PredicateExpression,
+            reference: ast.filter.PredicateExpression,
+            ) -> bool:
+        """Route *node* to the handler of the respective PredicateExpression subclass."""
+        if isinstance(reference, Any):
+            return True
+        if isinstance(node, ast.filter.Predicate):
+            return self._predicate(node, reference)
+        if isinstance(node, ast.filter.OneOf):
+            return self._one_of(node, reference)
+        # invalid node
+        raise errors.BackendError(f'expected predicate expression, found {node}')
+
+    def _one_of(self, node: ast.filter.OneOf, reference: ast.filter.PredicateExpression) -> bool:
+        if not isinstance(reference, type(node)):
+            return False
+        return _set_matcher(node, reference, self._parse_predicate_expression)
+
+    def _predicate(self, node: ast.filter.Predicate, reference: ast.filter.PredicateExpression) -> bool:
+        if not isinstance(reference, (Partial, type(node))):
+            return False
+        # partial check
+        if isinstance(reference, Partial):
+            if not isinstance(node, reference.node):
+                return False
+            return reference.match('predicate', node.predicate) \
+               and reference.match('reverse', node.reverse)
+        # full check
+        return node.predicate == reference.predicate \
+           and node.reverse == reference.reverse
+
+    def _branch(self,
+            node: typing.Union[ast.filter.Any, ast.filter.All],
+            reference: ast.filter.FilterExpression,
+            ) -> bool:
+        if not isinstance(reference, type(node)):
+            return False
+        if not self._parse_predicate_expression(node.predicate, reference.predicate): # type: ignore [attr-defined]
+            return False
+        if not self._parse_filter_expression(node.expr, reference.expr): # type: ignore [attr-defined]
+            return False
+        return True
+
+    def _agg(self, node: typing.Union[ast.filter.And, ast.filter.Or], reference: ast.filter.FilterExpression) -> bool:
+        if not isinstance(reference, type(node)):
+            return False
+        return _set_matcher(node, reference, self._parse_filter_expression) # type: ignore [arg-type]
+
+    def _not(self, node: ast.filter.Not, reference: ast.filter.FilterExpression) -> bool:
+        if not isinstance(reference, type(node)):
+            return False
+        return self._parse_filter_expression(node.expr, reference.expr)
+
+    def _has(self, node: ast.filter.Has, reference: ast.filter.FilterExpression) -> bool:
+        if not isinstance(reference, type(node)):
+            return False
+        return self._parse_predicate_expression(node.predicate, reference.predicate) \
+           and self._parse_filter_expression(node.count, reference.count)
+
+    def _distance(self, node: ast.filter.Distance, reference: ast.filter.FilterExpression) -> bool:
+        if not isinstance(reference, (Partial, type(node))):
+            return False
+        # partial check
+        if isinstance(reference, Partial):
+            if not isinstance(node, reference.node):
+                return False
+            return reference.match('reference', node.reference) \
+               and reference.match('threshold', node.threshold) \
+               and reference.match('strict', node.strict)
+        # full check
+        return node.reference == reference.reference \
+           and node.threshold == reference.threshold \
+           and node.strict == reference.strict
+
+    def _value(self, node: ast.filter._Value, reference: ast.filter.FilterExpression) -> bool:
+        if not isinstance(reference, (Partial, type(node))):
+            return False
+        # partial check
+        if isinstance(reference, Partial):
+            if not isinstance(node, reference.node):
+                return False
+            return reference.match('value', node.value)
+        # full ckeck
+        return node.value == reference.value
+
+    def _bounded(self, node: ast.filter._Bounded, reference: ast.filter.FilterExpression) -> bool:
+        if not isinstance(reference, (Partial, type(node))):
+            return False
+        # partial check
+        if isinstance(reference, Partial):
+            if not isinstance(node, reference.node):
+                return False
+            return reference.match('threshold', node.threshold) \
+               and reference.match('strict', node.strict)
+        # full check
+        return node.threshold == reference.threshold \
+           and node.strict == reference.strict
+
+## EOF ##
diff --git a/bsfs/query/validator.py b/bsfs/query/validator.py
new file mode 100644
index 0000000..10ca492
--- /dev/null
+++ b/bsfs/query/validator.py
@@ -0,0 +1,351 @@
+
+# imports
+import typing
+
+# bsfs imports
+from bsfs import schema as bsc
+from bsfs.namespace import ns
+from bsfs.utils import errors, typename
+
+# inner-module imports
+from . import ast
+
+# exports
+__all__ : typing.Sequence[str] = (
+    'Filter',
+    )
+
+# FIXME: Split into a submodule and the two classes into their own respective files.
+
+## code ##
+
+class Filter():
+    """Validate a `bsfs.query.ast.filter` query's structure and schema compliance.
+
+    * Conditions (Bounded, Value) can only be applied on literals
+    * Branches, Id, and Has can only be applied on nodes
+    * Predicates' domain and range must match
+    * Predicate paths must follow the schema
+    * Referenced types are present in the schema
+
+    """
+
+    # schema to validate against.
+    schema: bsc.Schema
+
+    def __init__(self, schema: bsc.Schema):
+        self.schema = schema
+
+    def __call__(self, root_type: bsc.Node, query: ast.filter.FilterExpression) -> bool:
+        """Alias for `Filter.validate`."""
+        return self.validate(root_type, query)
+
+    def validate(self, root_type: bsc.Node, query: ast.filter.FilterExpression) -> bool:
+        """Validate a filter *query*, assuming the subject having *root_type*.
+
+        Raises a `bsfs.utils.errors.ConsistencyError` if the query violates the schema.
+        Raises a `bsfs.utils.errors.BackendError` if the query structure is invalid.
+
+        """
+        # root_type must be a schema.Node
+        if not isinstance(root_type, bsc.Node):
+            raise TypeError(f'expected a node, found {typename(root_type)}')
+        # root_type must exist in the schema
+        if root_type not in self.schema.nodes():
+            raise errors.ConsistencyError(f'{root_type} is not defined in the schema')
+        # check root expression
+        self._parse_filter_expression(root_type, query)
+        # all tests passed
+        return True
+
+
+    ## routing methods
+
+    def _parse_filter_expression(self, type_: bsc.Vertex, node: ast.filter.FilterExpression):
+        """Route *node* to the handler of the respective FilterExpression subclass."""
+        if isinstance(node, ast.filter.Is):
+            return self._is(type_, node)
+        if isinstance(node, ast.filter.Not):
+            return self._not(type_, node)
+        if isinstance(node, ast.filter.Has):
+            return self._has(type_, node)
+        if isinstance(node, ast.filter.Distance):
+            return self._distance(type_, node)
+        if isinstance(node, (ast.filter.Any, ast.filter.All)):
+            return self._branch(type_, node)
+        if isinstance(node, (ast.filter.And, ast.filter.Or)):
+            return self._agg(type_, node)
+        if isinstance(node, (ast.filter.Equals, ast.filter.Substring, ast.filter.StartsWith, ast.filter.EndsWith)):
+            return self._value(type_, node)
+        if isinstance(node, (ast.filter.LessThan, ast.filter.GreaterThan)):
+            return self._bounded(type_, node)
+        # invalid node
+        raise errors.BackendError(f'expected filter expression, found {node}')
+
+    def _parse_predicate_expression(self, node: ast.filter.PredicateExpression) -> typing.Tuple[bsc.Vertex, bsc.Vertex]:
+        """Route *node* to the handler of the respective PredicateExpression subclass."""
+        if isinstance(node, ast.filter.Predicate):
+            return self._predicate(node)
+        if isinstance(node, ast.filter.OneOf):
+            return self._one_of(node)
+        # invalid node
+        raise errors.BackendError(f'expected predicate expression, found {node}')
+
+
+    ## predicate expressions
+
+    def _predicate(self, node: ast.filter.Predicate) -> typing.Tuple[bsc.Vertex, bsc.Vertex]:
+        # predicate exists in the schema
+        if not self.schema.has_predicate(node.predicate):
+            raise errors.ConsistencyError(f'predicate {node.predicate} is not in the schema')
+        # determine domain and range
+        pred = self.schema.predicate(node.predicate)
+        if not isinstance(pred.range, (bsc.Node, bsc.Literal)):
+            raise errors.BackendError(f'the range of predicate {pred} is undefined')
+        dom, rng = pred.domain, pred.range
+        if node.reverse:
+            dom, rng = rng, dom # type: ignore [assignment] # variable re-use confuses mypy
+        # return domain and range
+        return dom, rng
+
+    def _one_of(self, node: ast.filter.OneOf) -> typing.Tuple[bsc.Vertex, bsc.Vertex]:
+        # determine domain and range types
+        # NOTE: select the most specific domain and the most generic range
+        dom, rng = None, None
+        for pred in node:
+            # parse child expression
+            subdom, subrng = self._parse_predicate_expression(pred)
+            # determine overall domain
+            if dom is None or subdom < dom: # pick most specific domain
+                dom = subdom
+            # domains must be related across all child expressions
+            if not subdom <= dom and not subdom >= dom:
+                raise errors.ConsistencyError(f'domains {subdom} and {dom} are not related')
+            # determine overall range
+            if rng is None or subrng > rng: # pick most generic range
+                rng = subrng
+            # ranges must be related across all child expressions
+            if not subrng <= rng and not subrng >= rng:
+                raise errors.ConsistencyError(f'ranges {subrng} and {rng} are not related')
+        # OneOf guarantees at least one expression, dom and rng are always bsc.Vertex.
+        # mypy does not realize this, hence we ignore the warning.
+        return dom, rng # type: ignore [return-value]
+
+
+    ## intermediates
+
+    def _branch(self, type_: bsc.Vertex, node: ast.filter._Branch):
+        # type is a Node
+        if not isinstance(type_, bsc.Node):
+            raise errors.ConsistencyError(f'expected a Node, found {type_}')
+        # type exists in the schema
+        # FIXME: Isn't it actually guaranteed that the type (except the root type) is part of the schema?
+        # all types can be traced back to (a) root_type, (b) predicate, or (c) manually set (e.g. in _is).
+        # For (a), we do (and have to) perform a check. For (c), the code base should be consistent throughout
+        # the module, so this is an assumption that has to be ensured in schema.Schema. For (b), we know (and
+        # check) that the predicate is in the schema, hence all node/literals derived from it are also in the
+        # schema by construction of the schema.Schema class. So, why do we check this every time?
+        if type_ not in self.schema.nodes():
+            raise errors.ConsistencyError(f'node {type_} is not in the schema')
+        # predicate is valid
+        dom, rng = self._parse_predicate_expression(node.predicate)
+        # type_ is a subtype of the predicate's domain
+        if not type_ <= dom:
+            raise errors.ConsistencyError(f'expected type {dom} or subtype thereof, found {type_}')
+        # child expression is valid
+        self._parse_filter_expression(rng, node.expr)
+
+    def _agg(self, type_: bsc.Vertex, node: ast.filter._Agg):
+        for expr in node:
+            # child expression is valid
+            self._parse_filter_expression(type_, expr)
+
+    def _not(self, type_: bsc.Vertex, node: ast.filter.Not):
+        # child expression is valid
+        self._parse_filter_expression(type_, node.expr)
+
+    def _has(self, type_: bsc.Vertex, node: ast.filter.Has):
+        # type is a Node
+        if not isinstance(type_, bsc.Node):
+            raise errors.ConsistencyError(f'expected a Node, found {type_}')
+        # type exists in the schema
+        if type_ not in self.schema.nodes():
+            raise errors.ConsistencyError(f'node {type_} is not in the schema')
+        # predicate is valid
+        dom, _= self._parse_predicate_expression(node.predicate)
+        # type_ is a subtype of the predicate's domain
+        if not type_ <= dom:
+            raise errors.ConsistencyError(f'expected type {dom}, found {type_}')
+        # node.count is a numerical expression
+        self._parse_filter_expression(self.schema.literal(ns.bsl.Number), node.count)
+
+    def _distance(self, type_: bsc.Vertex, node: ast.filter.Distance):
+        # type is a Literal
+        if not isinstance(type_, bsc.Feature):
+            raise errors.ConsistencyError(f'expected a Feature, found {type_}')
+        # type exists in the schema
+        if type_ not in self.schema.literals():
+            raise errors.ConsistencyError(f'literal {type_} is not in the schema')
+        # reference matches type_
+        if len(node.reference) != type_.dimension:
+            raise errors.ConsistencyError(f'reference has dimension {len(node.reference)}, expected {type_.dimension}')
+        # FIXME: test dtype
+
+
+    ## conditions
+
+    def _is(self, type_: bsc.Vertex, node: ast.filter.Is): # pylint: disable=unused-argument # (node)
+        if not isinstance(type_, bsc.Node):
+            raise errors.ConsistencyError(f'expected a Node, found {type_}')
+        if type_ not in self.schema.nodes():
+            raise errors.ConsistencyError(f'node {type_} is not in the schema')
+
+    def _value(self, type_: bsc.Vertex, node: ast.filter._Value): # pylint: disable=unused-argument # (node)
+        # type is a literal
+        if not isinstance(type_, bsc.Literal):
+            raise errors.ConsistencyError(f'expected a Literal, found {type_}')
+        # type exists in the schema
+        if type_ not in self.schema.literals():
+            raise errors.ConsistencyError(f'literal {type_} is not in the schema')
+        # FIXME: Check if node.value corresponds to type_
+        # FIXME: A specific literal might be requested (i.e., a numeric type when used in Has)
+
+    def _bounded(self, type_: bsc.Vertex, node: ast.filter._Bounded): # pylint: disable=unused-argument # (node)
+        # type is a literal
+        if not isinstance(type_, bsc.Literal):
+            raise errors.ConsistencyError(f'expected a Literal, found {type_}')
+        # type exists in the schema
+        if type_ not in self.schema.literals():
+            raise errors.ConsistencyError(f'literal {type_} is not in the schema')
+        # type must be a numerical
+        if not type_ <= self.schema.literal(ns.bsl.Number):
+            raise errors.ConsistencyError(f'expected a number type, found {type_}')
+        # FIXME: Check if node.value corresponds to type_
+
+
+class Fetch():
+    """Validate a `bsfs.query.ast.fetch` query's structure and schema compliance.
+
+    * Value can only be applied on literals
+    * Node can only be applied on nodes
+    * Names must be non-empty
+    * Branching nodes' predicates must match the type
+    * Symbols must be in the schema
+    * Predicates must follow the schema
+
+    """
+
+    # schema to validate against.
+    schema: bsc.Schema
+
+    def __init__(self, schema: bsc.Schema):
+        self.schema = schema
+
+    def __call__(self, root_type: bsc.Node, query: ast.fetch.FetchExpression) -> bool:
+        """Alias for `Fetch.validate`."""
+        return self.validate(root_type, query)
+
+    def validate(self, root_type: bsc.Node, query: ast.fetch.FetchExpression) -> bool:
+        """Validate a fetch *query*, assuming the subject having *root_type*.
+
+        Raises a `bsfs.utils.errors.ConsistencyError` if the query violates the schema.
+        Raises a `bsfs.utils.errors.BackendError` if the query structure is invalid.
+
+        """
+        # root_type must be a schema.Node
+        if not isinstance(root_type, bsc.Node):
+            raise TypeError(f'expected a node, found {typename(root_type)}')
+        # root_type must exist in the schema
+        if root_type not in self.schema.nodes():
+            raise errors.ConsistencyError(f'{root_type} is not defined in the schema')
+        # query must be a FetchExpression
+        if not isinstance(query, ast.fetch.FetchExpression):
+            raise TypeError(f'expected a fetch expression, found {typename(query)}')
+        # check root expression
+        self._parse_fetch_expression(root_type, query)
+        # all tests passed
+        return True
+
+    def _parse_fetch_expression(self, type_: bsc.Vertex, node: ast.fetch.FetchExpression):
+        """Route *node* to the handler of the respective FetchExpression subclass."""
+        if isinstance(node, (ast.fetch.Fetch, ast.fetch.Value, ast.fetch.Node)):
+            # NOTE: don't return so that checks below are executed
+            self._branch(type_, node)
+        if isinstance(node, (ast.fetch.Value, ast.fetch.Node)):
+            # NOTE: don't return so that checks below are executed
+            self._named(type_, node)
+        if isinstance(node, ast.fetch.All):
+            return self._all(type_, node)
+        if isinstance(node, ast.fetch.Fetch):
+            return self._fetch(type_, node)
+        if isinstance(node, ast.fetch.Value):
+            return self._value(type_, node)
+        if isinstance(node, ast.fetch.Node):
+            return self._node(type_, node)
+        if isinstance(node, ast.fetch.This):
+            return self._this(type_, node)
+        # invalid node
+        raise errors.BackendError(f'expected fetch expression, found {node}')
+
+    def _all(self, type_: bsc.Vertex, node: ast.fetch.All):
+        # check child expressions
+        for expr in node:
+            self._parse_fetch_expression(type_, expr)
+
+    def _branch(self, type_: bsc.Vertex, node: ast.fetch._Branch):
+        # type is a node
+        if not isinstance(type_, bsc.Node):
+            raise errors.ConsistencyError(f'expected a Node, found {type_}')
+        # node exists in the schema
+        if type_ not in self.schema.nodes():
+            raise errors.ConsistencyError(f'node {type_} is not in the schema')
+        # predicate exists in the schema
+        if not self.schema.has_predicate(node.predicate):
+            raise errors.ConsistencyError(f'predicate {node.predicate} is not in the schema')
+        pred = self.schema.predicate(node.predicate)
+        # type_ must be a subclass of domain
+        if not type_ <= pred.domain:
+            raise errors.ConsistencyError(
+                f'expected type {pred.domain} or subtype thereof, found {type_}')
+
+    def _fetch(self, type_: bsc.Vertex, node: ast.fetch.Fetch): # pylint: disable=unused-argument # type_ was considered in _branch
+        # range must be a node
+        rng = self.schema.predicate(node.predicate).range
+        if not isinstance(rng, bsc.Node):
+            raise errors.ConsistencyError(
+                f'expected the predicate\'s range to be a Node, found {rng}')
+        # child expression must be valid
+        self._parse_fetch_expression(rng, node.expr)
+
+    def _named(self, type_: bsc.Vertex, node: ast.fetch._Named): # pylint: disable=unused-argument # type_ was considered in _branch
+        # name must be set
+        if node.name.strip() == '':
+            raise errors.BackendError('node name cannot be empty')
+        # FIXME: check for double name use?
+
+    def _node(self, type_: bsc.Vertex, node: ast.fetch.Node): # pylint: disable=unused-argument # type_ was considered in _branch
+        # range must be a node
+        rng = self.schema.predicate(node.predicate).range
+        if not isinstance(rng, bsc.Node):
+            raise errors.ConsistencyError(
+                f'expected the predicate\'s range to be a Node, found {rng}')
+
+    def _value(self, type_: bsc.Vertex, node: ast.fetch.Value): # pylint: disable=unused-argument # type_ was considered in _branch
+        # range must be a literal
+        rng = self.schema.predicate(node.predicate).range
+        if not isinstance(rng, bsc.Literal):
+            raise errors.ConsistencyError(
+                f'expected the predicate\'s range to be a Literal, found {rng}')
+
+    def _this(self, type_: bsc.Vertex, node: ast.fetch.This):
+        # type is a node
+        if not isinstance(type_, bsc.Node):
+            raise errors.ConsistencyError(f'expected a Node, found {type_}')
+        # node exists in the schema
+        if type_ not in self.schema.nodes():
+            raise errors.ConsistencyError(f'node {type_} is not in the schema')
+        # name must be set
+        if node.name.strip() == '':
+            raise errors.BackendError('node name cannot be empty')
+
+## EOF ##
diff --git a/bsfs/schema/__init__.py b/bsfs/schema/__init__.py
index ad4d456..ca2e0cd 100644
--- a/bsfs/schema/__init__.py
+++ b/bsfs/schema/__init__.py
@@ -1,15 +1,15 @@
-"""
 
-Part of the BlackStar filesystem (bsfs) module.
-A copy of the license is provided with the project.
-Author: Matthias Baumgartner, 2022
-"""
 # imports
 import typing
 
 # inner-module imports
 from .schema import Schema
-from .types import Literal, Node, Predicate
+from .serialize import from_string, to_string
+from .types import Literal, Node, Predicate, Vertex, Feature, \
+    ROOT_VERTEX, ROOT_NODE, ROOT_LITERAL, \
+    ROOT_NUMBER, ROOT_TIME, \
+    ROOT_ARRAY, ROOT_FEATURE, \
+    ROOT_PREDICATE
 
 # exports
 __all__: typing.Sequence[str] = (
@@ -17,6 +17,8 @@ __all__: typing.Sequence[str] = (
     'Node',
     'Predicate',
     'Schema',
+    'from_string',
+    'to_string',
     )
 
 ## EOF ##
diff --git a/bsfs/schema/schema.py b/bsfs/schema/schema.py
index c5d4571..c104436 100644
--- a/bsfs/schema/schema.py
+++ b/bsfs/schema/schema.py
@@ -1,16 +1,9 @@
-"""
 
-Part of the BlackStar filesystem (bsfs) module.
-A copy of the license is provided with the project.
-Author: Matthias Baumgartner, 2022
-"""
 # imports
 from collections import abc, namedtuple
 import typing
-import rdflib
 
 # bsfs imports
-from bsfs.namespace import ns
 from bsfs.utils import errors, URI, typename
 
 # inner-module imports
@@ -51,11 +44,13 @@ class Schema():
 
     def __init__(
             self,
-            predicates: typing.Iterable[types.Predicate],
+            predicates: typing.Optional[typing.Iterable[types.Predicate]] = None,
             nodes: typing.Optional[typing.Iterable[types.Node]] = None,
             literals: typing.Optional[typing.Iterable[types.Literal]] = None,
             ):
         # materialize arguments
+        if predicates is None:
+            predicates = set()
         if nodes is None:
             nodes = set()
         if literals is None:
@@ -63,24 +58,41 @@ class Schema():
         nodes = set(nodes)
         literals = set(literals)
         predicates = set(predicates)
+
+        # add root types to the schema
+        nodes.add(types.ROOT_NODE)
+        literals.add(types.ROOT_LITERAL)
+        predicates.add(types.ROOT_PREDICATE)
+        # add minimally necessary types to the schema
+        literals.add(types.ROOT_BLOB)
+        literals.add(types.ROOT_NUMBER)
+        literals.add(types.ROOT_TIME)
+        literals.add(types.ROOT_ARRAY)
+        literals.add(types.ROOT_FEATURE)
+
+        # FIXME: ensure that types derive from the right root?
+
         # include parents in predicates set
         # TODO: review type annotations and ignores for python >= 3.11 (parents is _Type but should be typing.Self)
         predicates |= {par for pred in predicates for par in pred.parents()} # type: ignore [misc]
         # include predicate domain in nodes set
         nodes |= {pred.domain for pred in predicates}
         # include predicate range in nodes and literals sets
-        prange = {pred.range for pred in predicates if pred.range is not None}
+        prange = {pred.range for pred in predicates}
         nodes |= {vert for vert in prange if isinstance(vert, types.Node)}
         literals |= {vert for vert in prange if isinstance(vert, types.Literal)}
+        # NOTE: ROOT_PREDICATE has a Vertex as range which is neither in nodes nor literals
+        # FIXME: with the ROOT_VERTEX missing, the schema is not complete anymore!
+
         # include parents in nodes and literals sets
-        # NOTE: Must be done after predicate domain/range was handled
-        # so that their parents are included as well.
+        # NOTE: Must come after predicate domain/range was handled to have their parents as well.
         nodes |= {par for node in nodes for par in node.parents()} # type: ignore [misc]
         literals |= {par for lit in literals for par in lit.parents()} # type: ignore [misc]
         # assign members
         self._nodes = {node.uri: node for node in nodes}
         self._literals = {lit.uri: lit for lit in literals}
         self._predicates = {pred.uri: pred for pred in predicates}
+
         # verify unique uris
         if len(nodes) != len(self._nodes):
             raise errors.ConsistencyError('inconsistent nodes')
@@ -214,6 +226,7 @@ class Schema():
         >>> Schema.Union([a, b, c])
 
         """
+        # FIXME: copy type annotations?
         if len(args) == 0:
             raise TypeError('Schema.Union requires at least one argument (Schema or Iterable)')
         if isinstance(args[0], cls): # args is sequence of Schema instances
@@ -295,92 +308,8 @@ class Schema():
         """Return the Literal matching the *uri*."""
         return self._literals[uri]
 
-
-    ## constructors ##
-
-
-    @classmethod
-    def Empty(cls) -> 'Schema': # pylint: disable=invalid-name # capitalized classmethod
-        """Return a minimal Schema."""
-        node = types.Node(ns.bsfs.Node, None)
-        literal = types.Literal(ns.bsfs.Literal, None)
-        predicate = types.Predicate(
-            uri=ns.bsfs.Predicate,
-            parent=None,
-            domain=node,
-            range=None,
-            unique=False,
-            )
-        return cls((predicate, ), (node, ), (literal, ))
-
-
-    @classmethod
-    def from_string(cls, schema: str) -> 'Schema': # pylint: disable=invalid-name # capitalized classmethod
-        """Load and return a Schema from a string."""
-        # parse string into rdf graph
-        graph = rdflib.Graph()
-        graph.parse(data=schema, format='turtle')
-
-        def _fetch_hierarchically(factory, curr):
-            # emit current node
-            yield curr
-            # walk through childs
-            for child in graph.subjects(rdflib.URIRef(ns.rdfs.subClassOf), rdflib.URIRef(curr.uri)):
-                # convert to URI
-                child = URI(child)
-                # check circular dependency
-                if child == curr.uri or child in {node.uri for node in curr.parents()}:
-                    raise errors.ConsistencyError('circular dependency')
-                # recurse and emit (sub*)childs
-                yield from _fetch_hierarchically(factory, factory(child, curr))
-
-        # fetch nodes
-        nodes = set(_fetch_hierarchically(types.Node, types.Node(ns.bsfs.Node, None)))
-        nodes_lut = {node.uri: node for node in nodes}
-        if len(nodes_lut) != len(nodes):
-            raise errors.ConsistencyError('inconsistent nodes')
-
-        # fetch literals
-        literals = set(_fetch_hierarchically(types.Literal, types.Literal(ns.bsfs.Literal, None)))
-        literals_lut = {lit.uri: lit for lit in literals}
-        if len(literals_lut) != len(literals):
-            raise errors.ConsistencyError('inconsistent literals')
-
-        # fetch predicates
-        def build_predicate(uri, parent):
-            uri = rdflib.URIRef(uri)
-            # get domain
-            domains = set(graph.objects(uri, rdflib.RDFS.domain))
-            if len(domains) != 1:
-                raise errors.ConsistencyError(f'inconsistent domain: {domains}')
-            dom = nodes_lut.get(next(iter(domains)))
-            if dom is None:
-                raise errors.ConsistencyError('missing domain')
-            # get range
-            ranges = set(graph.objects(uri, rdflib.RDFS.range))
-            if len(ranges) != 1:
-                raise errors.ConsistencyError(f'inconsistent range: {ranges}')
-            rng = next(iter(ranges))
-            rng = nodes_lut.get(rng, literals_lut.get(rng))
-            if rng is None:
-                raise errors.ConsistencyError('missing range')
-            # get unique flag
-            uniques = set(graph.objects(uri, rdflib.URIRef(ns.bsfs.unique)))
-            if len(uniques) != 1:
-                raise errors.ConsistencyError(f'inconsistent unique flags: {uniques}')
-            unique = bool(next(iter(uniques)))
-            # build Predicate
-            return types.Predicate(URI(uri), parent, dom, rng, unique)
-
-        root_predicate = types.Predicate(
-            uri=ns.bsfs.Predicate,
-            parent=None,
-            domain=nodes_lut[ns.bsfs.Node],
-            range=None, # FIXME: Unclear how to handle this! Can be either a Literal or a Node
-            unique=False,
-            )
-        predicates = _fetch_hierarchically(build_predicate, root_predicate)
-        # return Schema
-        return cls(predicates, nodes, literals)
+    def predicates_at(self, node: types.Node) -> typing.Iterator[types.Predicate]:
+        """Return predicates that have domain *node* (or superclass thereof)."""
+        return iter(pred for pred in self._predicates.values() if node <= pred.domain)
 
 ## EOF ##
diff --git a/bsfs/schema/serialize.py b/bsfs/schema/serialize.py
new file mode 100644
index 0000000..ea8b2f4
--- /dev/null
+++ b/bsfs/schema/serialize.py
@@ -0,0 +1,255 @@
+
+# standard imports
+import itertools
+import typing
+
+# external imports
+import rdflib
+
+# bsfs imports
+from bsfs.namespace import ns
+from bsfs.utils import errors, URI, typename
+
+# inner-module imports
+from . import types
+from . import schema
+
+# exports
+__all__: typing.Sequence[str] = (
+    'to_string',
+    'from_string',
+    )
+
+
+## code ##
+
+def from_string(schema_str: str) -> schema.Schema:
+    """Load and return a Schema from a string."""
+    # parse string into rdf graph
+    graph = rdflib.Graph()
+    graph.parse(data=schema_str, format='turtle')
+
+    # helper functions
+    def _fetch_value(
+            subject: URI,
+            predicate: rdflib.URIRef,
+            value_factory: typing.Callable[[typing.Any], typing.Any],
+            ) -> typing.Optional[typing.Any]:
+        """Fetch the object of a given subject and predicate.
+        Raises a `errors.ConsistencyError` if multiple objects match.
+        """
+        values = list(graph.objects(rdflib.URIRef(subject), predicate))
+        if len(values) == 0:
+            return None
+        if len(values) == 1:
+            return value_factory(values[0])
+        raise errors.ConsistencyError(
+            f'{subject} has multiple values for predicate {str(predicate)}, expected zero or one')
+
+    def _convert(value):
+        """Convert the subject type from rdflib to a bsfs native type."""
+        if isinstance(value, rdflib.Literal):
+            return value.value
+        if isinstance(value, rdflib.URIRef):
+            return URI(value)
+        # value is neither a node nor a literal, but e.g. a blank node
+        raise errors.BackendError(f'expected Literal or URIRef, found {typename(value)}')
+
+    def _fetch_hierarchically(factory, curr):
+        """Walk through a rdfs:subClassOf hierarchy, creating symbols along the way."""
+        # emit current node
+        yield curr
+        # walk through childs
+        for child in graph.subjects(rdflib.URIRef(ns.rdfs.subClassOf), rdflib.URIRef(curr.uri)):
+            # fetch annotations
+            annotations = {
+                URI(pred): _convert(value)
+                for pred, value # FIXME: preserve datatype of value?!
+                in graph.predicate_objects(child)
+                if URI(pred) != ns.rdfs.subClassOf
+                }
+            # convert child to URI
+            child = URI(child)
+            # check circular dependency
+            if child == curr.uri or child in {node.uri for node in curr.parents()}:
+                raise errors.ConsistencyError('circular dependency')
+            # recurse and emit (sub*)childs
+            yield from _fetch_hierarchically(factory, factory(child, curr, **annotations))
+
+    # fetch nodes
+    nodes = set(_fetch_hierarchically(types.Node, types.ROOT_NODE))
+    nodes_lut = {node.uri: node for node in nodes}
+    if len(nodes_lut) != len(nodes):
+        raise errors.ConsistencyError('inconsistent nodes')
+
+    # fetch literals
+    def _build_literal(uri, parent, **annotations):
+        """Literal factory."""
+        # break out on root feature type
+        if uri == types.ROOT_FEATURE.uri:
+            return types.ROOT_FEATURE
+        # handle feature types
+        if isinstance(parent, types.Feature):
+            # clean annotations
+            annotations.pop(ns.bsfs.dimension, None)
+            annotations.pop(ns.bsfs.dtype, None)
+            annotations.pop(ns.bsfs.distance, None)
+            # get dimension
+            dimension = _fetch_value(uri, rdflib.URIRef(ns.bsfs.dimension), int)
+            # get dtype
+            dtype = _fetch_value(uri, rdflib.URIRef(ns.bsfs.dtype), URI)
+            # get distance
+            distance = _fetch_value(uri, rdflib.URIRef(ns.bsfs.distance), URI)
+            # return feature
+            return parent.child(URI(uri), dtype=dtype, dimension=dimension, distance=distance, **annotations)
+        # handle non-feature types
+        return parent.child(URI(uri), **annotations)
+
+    literals = set(_fetch_hierarchically(_build_literal, types.ROOT_LITERAL))
+    literals_lut = {lit.uri: lit for lit in literals}
+    if len(literals_lut) != len(literals):
+        raise errors.ConsistencyError('inconsistent literals')
+
+    # fetch predicates
+    def _build_predicate(uri, parent, **annotations):
+        """Predicate factory."""
+        # clean annotations
+        annotations.pop(ns.rdfs.domain, None)
+        annotations.pop(ns.rdfs.range, None)
+        annotations.pop(ns.bsfs.unique, None)
+        # get domain
+        dom = _fetch_value(uri, rdflib.RDFS.domain, URI)
+        if dom is not None and dom not in nodes_lut:
+            raise errors.ConsistencyError(f'predicate {uri} has undefined domain {dom}')
+        if dom is not None:
+            dom = nodes_lut[dom]
+        # get range
+        rng = _fetch_value(uri, rdflib.RDFS.range, URI)
+        if rng is not None and rng not in nodes_lut and rng not in literals_lut:
+            raise errors.ConsistencyError(f'predicate {uri} has undefined range {rng}')
+        if rng is not None:
+            rng = nodes_lut.get(rng, literals_lut.get(rng))
+        # get unique
+        unique = _fetch_value(uri, rdflib.URIRef(ns.bsfs.unique), bool)
+        # build predicate
+        return parent.child(URI(uri), domain=dom, range=rng, unique=unique, **annotations)
+
+    predicates = _fetch_hierarchically(_build_predicate, types.ROOT_PREDICATE)
+
+    return schema.Schema(predicates, nodes, literals)
+
+
+
+def to_string(schema_inst: schema.Schema, fmt: str = 'turtle') -> str:
+    """Serialize a `bsfs.schema.Schema` to a string.
+    See `rdflib.Graph.serialize` for viable formats (default: turtle).
+    """
+
+    # type of emitted triples.
+    T_TRIPLE = typing.Iterator[typing.Tuple[rdflib.URIRef, rdflib.URIRef, rdflib.term.Identifier]]
+
+    def _type(tpe: types._Type) -> T_TRIPLE :
+        """Emit _Type properties (parent, annotations)."""
+        # emit parent
+        if tpe.parent is not None:
+            yield (
+                rdflib.URIRef(tpe.uri),
+                rdflib.URIRef(ns.rdfs.subClassOf),
+                rdflib.URIRef(tpe.parent.uri),
+                )
+        # emit annotations
+        for prop, value in tpe.annotations.items():
+            yield (
+                rdflib.URIRef(tpe.uri),
+                rdflib.URIRef(prop),
+                rdflib.Literal(value), # FIXME: datatype?!
+                )
+
+    def _predicate(pred: types.Predicate) -> T_TRIPLE:
+        """Emit Predicate properties (domain, range, unique)."""
+        # no need to emit anything for the root predicate
+        if pred == types.ROOT_PREDICATE:
+            return
+        # emit domain
+        if pred.domain != getattr(pred.parent, 'domain', None):
+            yield (
+                rdflib.URIRef(pred.uri),
+                rdflib.URIRef(ns.rdfs.domain),
+                rdflib.URIRef(pred.domain.uri),
+                )
+        # emit range
+        if pred.range != getattr(pred.parent, 'range', None):
+            yield (
+                rdflib.URIRef(pred.uri),
+                rdflib.URIRef(ns.rdfs.range),
+                rdflib.URIRef(pred.range.uri),
+                )
+        # emit cardinality
+        if pred.unique != getattr(pred.parent, 'unique', None):
+            yield (
+                rdflib.URIRef(pred.uri),
+                rdflib.URIRef(ns.bsfs.unique),
+                rdflib.Literal(pred.unique, datatype=rdflib.XSD.boolean),
+                )
+
+    def _feature(feat: types.Feature) -> T_TRIPLE:
+        """Emit Feature properties (dimension, dtype, distance)."""
+        # emit size
+        if feat.dimension != getattr(feat.parent, 'dimension', None):
+            yield (
+                rdflib.URIRef(feat.uri),
+                rdflib.URIRef(ns.bsfs.dimension),
+                rdflib.Literal(feat.dimension, datatype=rdflib.XSD.integer),
+                )
+        # emit dtype
+        if feat.dtype != getattr(feat.parent, 'dtype', None):
+            yield (
+                rdflib.URIRef(feat.uri),
+                rdflib.URIRef(ns.bsfs.dtype),
+                rdflib.URIRef(feat.dtype),
+                )
+        # emit distance
+        if feat.distance != getattr(feat.parent, 'distance', None):
+            yield (
+                rdflib.URIRef(feat.uri),
+                rdflib.URIRef(ns.bsfs.distance),
+                rdflib.URIRef(feat.distance),
+                )
+
+    def _parse(node: types._Type) -> T_TRIPLE:
+        """Emit all properties of a type."""
+        # check arg
+        if not isinstance(node, types._Type): # pylint: disable=protected-access
+            raise TypeError(node)
+        # emit _Type essentials
+        yield from _type(node)
+        # emit properties of derived types
+        if isinstance(node, types.Predicate):
+            yield from _predicate(node)
+        if isinstance(node, types.Feature):
+            yield from _feature(node)
+
+    # create graph
+    graph = rdflib.Graph()
+    # add triples to graph
+    nodes = itertools.chain(
+        schema_inst.nodes(),
+        schema_inst.literals(),
+        schema_inst.predicates())
+    for node in nodes:
+        for triple in _parse(node):
+            graph.add(triple)
+    # add known namespaces for readability
+    # FIXME: more generically?
+    graph.bind('bsfs', rdflib.URIRef(ns.bsfs + '/'))
+    graph.bind('bsl', rdflib.URIRef(ns.bsl + '/'))
+    graph.bind('bsn', rdflib.URIRef(ns.bsn + '#'))
+    graph.bind('bse', rdflib.URIRef(ns.bsfs.Entity() + '#'))
+    graph.bind('rdf', rdflib.URIRef(ns.rdf))
+    graph.bind('rdfs', rdflib.URIRef(ns.rdfs))
+    graph.bind('schema', rdflib.URIRef(ns.schema))
+    graph.bind('xsd', rdflib.URIRef(ns.xsd))
+    # serialize to turtle
+    return graph.serialize(format=fmt)
+
+## EOF ##
diff --git a/bsfs/schema/types.py b/bsfs/schema/types.py
index 54a7e99..5834df8 100644
--- a/bsfs/schema/types.py
+++ b/bsfs/schema/types.py
@@ -1,13 +1,9 @@
-"""
 
-Part of the BlackStar filesystem (bsfs) module.
-A copy of the license is provided with the project.
-Author: Matthias Baumgartner, 2022
-"""
 # imports
 import typing
 
 # bsfs imports
+from bsfs.namespace import ns
 from bsfs.utils import errors, URI, typename
 
 # exports
@@ -15,6 +11,7 @@ __all__: typing.Sequence[str] = (
     'Literal',
     'Node',
     'Predicate',
+    'Feature',
     )
 
 
@@ -99,9 +96,11 @@ class _Type():
             self,
             uri: URI,
             parent: typing.Optional['_Type'] = None,
+            **annotations: typing.Any,
             ):
-        self.uri = uri
+        self.uri = URI(uri)
         self.parent = parent
+        self.annotations = annotations
 
     def parents(self) -> typing.Generator['_Type', None, None]:
         """Generate a list of parent nodes."""
@@ -110,9 +109,17 @@ class _Type():
             yield curr
             curr = curr.parent
 
-    def get_child(self, uri: URI, **kwargs):
+    def child(
+            self,
+            uri: URI,
+            **kwargs,
+            ):
         """Return a child of the current class."""
-        return type(self)(uri, self, **kwargs)
+        return type(self)(
+            uri=uri,
+            parent=self,
+            **kwargs
+            )
 
     def __str__(self) -> str:
         return f'{typename(self)}({self.uri})'
@@ -138,8 +145,10 @@ class _Type():
 
     def __lt__(self, other: typing.Any) -> bool:
         """Return True iff *self* is a true subclass of *other*."""
-        if not type(self) is type(other): # type mismatch # pylint: disable=unidiomatic-typecheck
+        if not isinstance(other, _Type):
             return NotImplemented
+        if not isinstance(other, type(self)): # FIXME: necessary?
+            return False
         if self.uri == other.uri: # equivalence
             return False
         if self in other.parents(): # superclass
@@ -151,8 +160,10 @@ class _Type():
 
     def __le__(self, other: typing.Any) -> bool:
         """Return True iff *self* is equivalent or a subclass of *other*."""
-        if not type(self) is type(other): # type mismatch # pylint: disable=unidiomatic-typecheck
+        if not isinstance(other, _Type):
             return NotImplemented
+        if not isinstance(other, type(self)): # FIXME: necessary?
+            return False
         if self.uri == other.uri: # equivalence
             return True
         if self in other.parents(): # superclass
@@ -164,8 +175,10 @@ class _Type():
 
     def __gt__(self, other: typing.Any) -> bool:
         """Return True iff *self* is a true superclass of *other*."""
-        if not type(self) is type(other): # type mismatch # pylint: disable=unidiomatic-typecheck
+        if not isinstance(other, _Type):
             return NotImplemented
+        if not isinstance(other, type(self)): # FIXME: necessary?
+            return False
         if self.uri == other.uri: # equivalence
             return False
         if self in other.parents(): # superclass
@@ -177,8 +190,10 @@ class _Type():
 
     def __ge__(self, other: typing.Any) -> bool:
         """Return True iff *self* is eqiuvalent or a superclass of *other*."""
-        if not type(self) is type(other): # type mismatch # pylint: disable=unidiomatic-typecheck
+        if not isinstance(other, _Type):
             return NotImplemented
+        if not isinstance(other, type(self)): # FIXME: necessary?
+            return False
         if self.uri == other.uri: # equivalence
             return True
         if self in other.parents(): # superclass
@@ -189,32 +204,95 @@ class _Type():
         return False
 
 
-class _Vertex(_Type):
+class Vertex(_Type):
     """Graph vertex types. Can be a Node or a Literal."""
-    def __init__(self, uri: URI, parent: typing.Optional['_Vertex']):
-        super().__init__(uri, parent)
+    parent: typing.Optional['Vertex']
+    def __init__(self, uri: URI, parent: typing.Optional['Vertex'], **kwargs):
+        super().__init__(uri, parent, **kwargs)
 
 
-class Node(_Vertex):
+class Node(Vertex):
     """Node type."""
-    def __init__(self, uri: URI, parent: typing.Optional['Node']):
-        super().__init__(uri, parent)
+    parent: typing.Optional['Node']
+    def __init__(self, uri: URI, parent: typing.Optional['Node'], **kwargs):
+        super().__init__(uri, parent, **kwargs)
 
 
-class Literal(_Vertex):
+class Literal(Vertex):
     """Literal type."""
-    def __init__(self, uri: URI, parent: typing.Optional['Literal']):
-        super().__init__(uri, parent)
+    parent: typing.Optional['Literal']
+    def __init__(self, uri: URI, parent: typing.Optional['Literal'], **kwargs):
+        super().__init__(uri, parent, **kwargs)
+
+
+class Feature(Literal):
+    """Feature type."""
+
+    # Number of feature vector dimensions.
+    dimension: int
+
+    # Feature vector datatype.
+    dtype: URI
+
+    # Distance measure to compare feature vectors.
+    distance: URI
+
+    def __init__(
+            self,
+            # Type members
+            uri: URI,
+            parent: typing.Optional[Literal],
+            # Feature members
+            dimension: int,
+            dtype: URI,
+            distance: URI,
+            **kwargs,
+            ):
+        super().__init__(uri, parent, **kwargs)
+        self.dimension = int(dimension)
+        self.dtype = URI(dtype)
+        self.distance = URI(distance)
+
+    def __hash__(self) -> int:
+        return hash((super().__hash__(), self.dimension, self.dtype, self.distance))
+
+    def __eq__(self, other: typing.Any) -> bool:
+        return super().__eq__(other) \
+           and self.dimension == other.dimension \
+           and self.dtype == other.dtype \
+           and self.distance == other.distance
 
+    def child(
+            self,
+            uri: URI,
+            dimension: typing.Optional[int] = None,
+            dtype: typing.Optional[URI] = None,
+            distance: typing.Optional[URI] = None,
+            **kwargs,
+            ):
+        """Return a child of the current class."""
+        if dimension is None:
+            dimension = self.dimension
+        if dtype is None:
+            dtype = self.dtype
+        if distance is None:
+            distance = self.distance
+        return super().child(
+            uri=uri,
+            dimension=dimension,
+            dtype=dtype,
+            distance=distance,
+            **kwargs,
+            )
 
 class Predicate(_Type):
-    """Predicate type."""
+    """Predicate base type."""
 
     # source type.
     domain: Node
 
     # destination type.
-    range: typing.Optional[typing.Union[Node, Literal]]
+    range: Vertex
 
     # maximum cardinality of type.
     unique: bool
@@ -226,22 +304,23 @@ class Predicate(_Type):
             parent: typing.Optional['Predicate'],
             # Predicate members
             domain: Node,
-            range: typing.Optional[typing.Union[Node, Literal]], # pylint: disable=redefined-builtin
+            range: Vertex, # pylint: disable=redefined-builtin
             unique: bool,
+            **kwargs,
             ):
         # check arguments
         if not isinstance(domain, Node):
             raise TypeError(domain)
-        if range is not None and not isinstance(range, Node) and not isinstance(range, Literal):
+        if range != ROOT_VERTEX and not isinstance(range, (Node, Literal)):
             raise TypeError(range)
         # initialize
-        super().__init__(uri, parent)
+        super().__init__(uri, parent, **kwargs)
         self.domain = domain
         self.range = range
-        self.unique = unique
+        self.unique = bool(unique)
 
     def __hash__(self) -> int:
-        return hash((super().__hash__(), self.domain, self.range, self.unique))
+        return hash((super().__hash__(), self.domain, self.unique, self.range))
 
     def __eq__(self, other: typing.Any) -> bool:
         return super().__eq__(other) \
@@ -249,11 +328,11 @@ class Predicate(_Type):
            and self.range == other.range \
            and self.unique == other.unique
 
-    def get_child(
+    def child(
             self,
             uri: URI,
             domain: typing.Optional[Node] = None,
-            range: typing.Optional[_Vertex] = None, # pylint: disable=redefined-builtin
+            range: typing.Optional[Vertex] = None, # pylint: disable=redefined-builtin
             unique: typing.Optional[bool] = None,
             **kwargs,
             ):
@@ -264,13 +343,73 @@ class Predicate(_Type):
             raise errors.ConsistencyError(f'{domain} must be a subclass of {self.domain}')
         if range is None:
             range = self.range
-        if range is None: # inherited range from ns.bsfs.Predicate
-            raise ValueError('range must be defined by the parent or argument')
-        if self.range is not None and not range <= self.range:
+        # NOTE: The root predicate has a Vertex as range, which is neither a parent of the root
+        # Node nor Literal. Hence, that test is skipped since a child should be allowed to
+        # specialize from Vertex to anything.
+        if self.range != ROOT_VERTEX and not range <= self.range:
             raise errors.ConsistencyError(f'{range} must be a subclass of {self.range}')
         if unique is None:
             unique = self.unique
-        return super().get_child(uri, domain=domain, range=range, unique=unique, **kwargs)
+        return super().child(
+            uri=uri,
+            domain=domain,
+            range=range,
+            unique=unique,
+            **kwargs
+            )
+
+
+# essential vertices
+ROOT_VERTEX = Vertex(
+    uri=ns.bsfs.Vertex,
+    parent=None,
+    )
+
+ROOT_NODE = Node(
+    uri=ns.bsfs.Node,
+    parent=None,
+    )
+
+ROOT_LITERAL = Literal(
+    uri=ns.bsfs.Literal,
+    parent=None,
+    )
+
+ROOT_BLOB = Literal(
+    uri=ns.bsl.BinaryBlob,
+    parent=ROOT_LITERAL,
+    )
+
+ROOT_NUMBER = Literal(
+    uri=ns.bsl.Number,
+    parent=ROOT_LITERAL,
+    )
+
+ROOT_TIME = Literal(
+    uri=ns.bsl.Time,
+    parent=ROOT_LITERAL,
+    )
+
+ROOT_ARRAY = Literal(
+    uri=ns.bsl.Array,
+    parent=ROOT_LITERAL,
+    )
 
+ROOT_FEATURE = Feature(
+    uri=ns.bsl.Array.Feature,
+    parent=ROOT_ARRAY,
+    dimension=1,
+    dtype=ns.bsfs.dtype().f16,
+    distance=ns.bsd.euclidean,
+    )
+
+# essential predicates
+ROOT_PREDICATE = Predicate(
+    uri=ns.bsfs.Predicate,
+    parent=None,
+    domain=ROOT_NODE,
+    range=ROOT_VERTEX,
+    unique=False,
+    )
 
 ## EOF ##
diff --git a/bsfs/triple_store/__init__.py b/bsfs/triple_store/__init__.py
index fb5a8a9..79a2887 100644
--- a/bsfs/triple_store/__init__.py
+++ b/bsfs/triple_store/__init__.py
@@ -1,9 +1,4 @@
-"""
 
-Part of the BlackStar filesystem (bsfs) module.
-A copy of the license is provided with the project.
-Author: Matthias Baumgartner, 2022
-"""
 # imports
 import typing
 
diff --git a/bsfs/triple_store/base.py b/bsfs/triple_store/base.py
index 6561262..58b5670 100644
--- a/bsfs/triple_store/base.py
+++ b/bsfs/triple_store/base.py
@@ -1,16 +1,12 @@
-"""
 
-Part of the BlackStar filesystem (bsfs) module.
-A copy of the license is provided with the project.
-Author: Matthias Baumgartner, 2022
-"""
 # imports
 import abc
 import typing
 
 # inner-module imports
+from bsfs.query import ast
 from bsfs.utils import URI, typename
-import bsfs.schema  as _schema
+import bsfs.schema  as bsc
 
 # exports
 __all__: typing.Sequence[str] = (
@@ -81,12 +77,12 @@ class TripleStoreBase(abc.ABC):
 
     @property
     @abc.abstractmethod
-    def schema(self) -> _schema.Schema:
+    def schema(self) -> bsc.Schema:
         """Return the store's local schema."""
 
     @schema.setter
     @abc.abstractmethod
-    def schema(self, schema: _schema.Schema):
+    def schema(self, schema: bsc.Schema):
         """Migrate to new schema by adding or removing class definitions.
 
         Commits before and after the migration.
@@ -109,9 +105,30 @@ class TripleStoreBase(abc.ABC):
         """
 
     @abc.abstractmethod
+    def get(
+            self,
+            node_type: bsc.Node,
+            filter: typing.Optional[ast.filter.FilterExpression] = None, # pylint: disable=redefined-builtin
+            ) -> typing.Iterator[URI]:
+        """Return guids of nodes of type *node_type* that match the *filter*.
+        Return all guids of the respective type if *filter* is None.
+        """
+
+    @abc.abstractmethod
+    def fetch(
+            self,
+            node_type: bsc.Node,
+            filter: ast.filter.FilterExpression, # pylint: disable=redefined-builtin
+            fetch: ast.fetch.FetchExpression,
+            ) -> typing.Iterator[typing.Tuple[URI, str, typing.Any]]:
+        """Return (guid, name, value) triples where the guid is determined by the *filter*
+        query and the name matches the *fetch* query.
+        """
+
+    @abc.abstractmethod
     def exists(
             self,
-            node_type: _schema.Node,
+            node_type: bsc.Node,
             guids: typing.Iterable[URI],
             ) -> typing.Iterable[URI]:
         """Return those *guids* that exist and have type *node_type* or a subclass thereof."""
@@ -119,7 +136,7 @@ class TripleStoreBase(abc.ABC):
     @abc.abstractmethod
     def create(
             self,
-            node_type: _schema.Node,
+            node_type: bsc.Node,
             guids: typing.Iterable[URI],
             ):
         """Create *guid* nodes with type *subject*."""
@@ -127,9 +144,9 @@ class TripleStoreBase(abc.ABC):
     @abc.abstractmethod
     def set(
             self,
-            node_type: _schema.Node, # FIXME: is the node_type even needed? Couldn't I infer from the predicate?
+            node_type: bsc.Node, # FIXME: is the node_type even needed? Couldn't I infer from the predicate?
             guids: typing.Iterable[URI],
-            predicate: _schema.Predicate,
+            predicate: bsc.Predicate,
             values: typing.Iterable[typing.Any],
             ):
         """Add triples to the graph.
diff --git a/bsfs/triple_store/sparql/__init__.py b/bsfs/triple_store/sparql/__init__.py
new file mode 100644
index 0000000..cfa2732
--- /dev/null
+++ b/bsfs/triple_store/sparql/__init__.py
@@ -0,0 +1,13 @@
+
+# imports
+import typing
+
+# inner-module imports
+from .sparql import SparqlStore
+
+# exports
+__all__: typing.Sequence[str] = (
+    'SparqlStore',
+    )
+
+## EOF ##
diff --git a/bsfs/triple_store/sparql/distance.py b/bsfs/triple_store/sparql/distance.py
new file mode 100644
index 0000000..2c2f355
--- /dev/null
+++ b/bsfs/triple_store/sparql/distance.py
@@ -0,0 +1,51 @@
+
+# standard imports
+import typing
+
+# external imports
+import numpy as np
+
+# bsfs imports
+from bsfs.namespace import ns
+
+# constants
+EPS = 1e-9
+
+# exports
+__all__: typing.Sequence[str] = (
+    'DISTANCE_FU',
+    )
+
+
+## code ##
+
+def euclid(fst, snd) -> float:
+    """Euclidean distance (l2 norm)."""
+    fst = np.array(fst)
+    snd = np.array(snd)
+    return float(np.linalg.norm(fst - snd))
+
+def cosine(fst, snd) -> float:
+    """Cosine distance."""
+    fst = np.array(fst)
+    snd = np.array(snd)
+    if (fst == snd).all():
+        return 0.0
+    nrm0 = np.linalg.norm(fst)
+    nrm1 = np.linalg.norm(snd)
+    return float(1.0 - np.dot(fst, snd) / (nrm0 * nrm1 + EPS))
+
+def manhatten(fst, snd) -> float:
+    """Manhatten (cityblock) distance (l1 norm)."""
+    fst = np.array(fst)
+    snd = np.array(snd)
+    return float(np.abs(fst - snd).sum())
+
+# Known distance functions.
+DISTANCE_FU = {
+    ns.bsd.euclidean: euclid,
+    ns.bsd.cosine: cosine,
+    ns.bsd.manhatten: manhatten,
+}
+
+## EOF ##
diff --git a/bsfs/triple_store/sparql/parse_fetch.py b/bsfs/triple_store/sparql/parse_fetch.py
new file mode 100644
index 0000000..fab8173
--- /dev/null
+++ b/bsfs/triple_store/sparql/parse_fetch.py
@@ -0,0 +1,104 @@
+
+# standard imports
+import typing
+
+# bsfs imports
+from bsfs import schema as bsc
+from bsfs.query import ast
+from bsfs.utils import errors
+
+# inner-module imports
+from .utils import GenHopName, Query
+
+# exports
+__all__: typing.Sequence[str] = (
+    'Fetch',
+    )
+
+
+## code ##
+
+class Fetch():
+    """Translate `bsfs.query.ast.fetch` structures into Sparql queries."""
+
+    def __init__(self, schema):
+        self.schema = schema
+        self.ngen = GenHopName(prefix='?fch')
+
+    def __call__(
+            self,
+            root_type: bsc.Node,
+            root: ast.fetch.FetchExpression,
+            ) -> Query:
+        """
+        """
+        # check root_type
+        if not isinstance(root_type, bsc.Node):
+            raise errors.BackendError(f'expected Node, found {root_type}')
+        if root_type not in self.schema.nodes():
+            raise errors.ConsistencyError(f'node {root_type} is not in the schema')
+        # parse root
+        terms, expr = self._parse_fetch_expression(root_type, root, '?ent')
+        # assemble query
+        return Query(
+            root_type=root_type.uri,
+            root_head='?ent',
+            select=terms,
+            where=expr,
+            )
+
+    def _parse_fetch_expression(
+            self,
+            node_type: bsc.Vertex,
+            node: ast.fetch.FetchExpression,
+            head: str,
+            ):
+        """Route *node* to the handler of the respective FetchExpression subclass."""
+        if isinstance(node, ast.fetch.All):
+            return self._all(node_type, node, head)
+        if isinstance(node, ast.fetch.Fetch):
+            return self._fetch(node_type, node, head)
+        if isinstance(node, ast.fetch.Node):
+            return self._node(node_type, node, head)
+        if isinstance(node, ast.fetch.Value):
+            return self._value(node_type, node, head)
+        if isinstance(node, ast.fetch.This):
+            return self._this(node_type, node, head)
+        # invalid node
+        raise errors.BackendError(f'expected fetch expression, found {node}')
+
+    def _all(self, node_type: bsc.Vertex, node: ast.fetch.All, head: str):
+        # child expressions
+        terms, exprs = zip(*[self._parse_fetch_expression(node_type, expr, head) for expr in node])
+        terms = {term for sub in terms for term in sub}
+        exprs = ' .\n'.join({expr for expr in exprs if len(expr.strip()) > 0})
+        return terms, exprs
+
+    def _fetch(self, node_type: bsc.Vertex, node: ast.fetch.Fetch, head: str): # pylint: disable=unused-argument # (node_type)
+        # child expressions
+        rng = self.schema.predicate(node.predicate).range
+        nexthead = next(self.ngen)
+        terms, expr = self._parse_fetch_expression(rng, node.expr, nexthead)
+        return terms, f'OPTIONAL{{ {head} <{node.predicate}> {nexthead} .\n {expr} }}'
+
+    def _node(self, node_type: bsc.Vertex, node: ast.fetch.Node, head: str): # pylint: disable=unused-argument # (node_type)
+        if f'?{node.name}'.startswith(self.ngen.prefix):
+            raise errors.BackendError(f'Node name must start with {self.ngen.prefix}')
+        # compose and return statement
+        term = next(self.ngen)
+        return {(term, node.name)}, f'OPTIONAL{{ {head} <{node.predicate}> {term} }}'
+
+    def _value(self, node_type: bsc.Vertex, node: ast.fetch.Value, head: str): # pylint: disable=unused-argument # (node_type)
+        if f'?{node.name}'.startswith(self.ngen.prefix):
+            raise errors.BackendError(f'Value name must start with {self.ngen.prefix}')
+        # compose and return statement
+        term = next(self.ngen)
+        return {(term, node.name)}, f'OPTIONAL{{ {head} <{node.predicate}> {term} }}'
+
+    def _this(self, node_type: bsc.Vertex, node: ast.fetch.This, head: str): # pylint: disable=unused-argument # (node_type)
+        if f'?{node.name}'.startswith(self.ngen.prefix):
+            raise errors.BackendError(f'This name must start with {self.ngen.prefix}')
+        # compose and return statement
+        return {(head, node.name)}, ''
+
+## EOF ##
diff --git a/bsfs/triple_store/sparql/parse_filter.py b/bsfs/triple_store/sparql/parse_filter.py
new file mode 100644
index 0000000..2f5a25b
--- /dev/null
+++ b/bsfs/triple_store/sparql/parse_filter.py
@@ -0,0 +1,316 @@
+
+# imports
+import operator
+import typing
+
+# external imports
+import rdflib
+
+# bsfs imports
+from bsfs import schema as bsc
+from bsfs.namespace import ns
+from bsfs.query import ast
+from bsfs.utils import URI, errors
+
+# inner-module imports
+from .distance import DISTANCE_FU
+from .utils import GenHopName, Query
+
+# exports
+__all__: typing.Sequence[str] = (
+    'Filter',
+    )
+
+
+## code ##
+
+class Filter():
+    """Translate `bsfs.query.ast.filter` structures into Sparql queries."""
+
+    # Current schema to validate against.
+    schema: bsc.Schema
+
+    # Generator that produces unique symbol names.
+    ngen: GenHopName
+
+    def __init__(self, graph, schema):
+        self.graph = graph
+        self.schema = schema
+        self.ngen = GenHopName(prefix='?flt')
+
+    def __call__(
+            self,
+            root_type: bsc.Node,
+            root: typing.Optional[ast.filter.FilterExpression] = None,
+            ) -> Query:
+        """
+        """
+        # check root_type
+        if not isinstance(root_type, bsc.Node):
+            raise errors.BackendError(f'expected Node, found {root_type}')
+        if root_type not in self.schema.nodes():
+            raise errors.ConsistencyError(f'node {root_type} is not in the schema')
+        # parse root
+        if root is None:
+            cond = ''
+        else:
+            cond = self._parse_filter_expression(root_type, root, '?ent')
+        # assemble query
+        return Query(
+            root_type=root_type.uri,
+            root_head='?ent',
+            where=cond,
+            )
+
+    def _parse_filter_expression(
+            self,
+            type_: bsc.Vertex,
+            node: ast.filter.FilterExpression,
+            head: str,
+            ) -> str:
+        """Route *node* to the handler of the respective FilterExpression subclass."""
+        if isinstance(node, ast.filter.Is):
+            return self._is(type_, node, head)
+        if isinstance(node, ast.filter.Not):
+            return self._not(type_, node, head)
+        if isinstance(node, ast.filter.Has):
+            return self._has(type_, node, head)
+        if isinstance(node, ast.filter.Distance):
+            return self._distance(type_, node, head)
+        if isinstance(node, ast.filter.Any):
+            return self._any(type_, node, head)
+        if isinstance(node, ast.filter.All):
+            return self._all(type_, node, head)
+        if isinstance(node, ast.filter.And):
+            return self._and(type_, node, head)
+        if isinstance(node, ast.filter.Or):
+            return self._or(type_, node, head)
+        if isinstance(node, ast.filter.Equals):
+            return self._equals(type_, node, head)
+        if isinstance(node, ast.filter.Substring):
+            return self._substring(type_, node, head)
+        if isinstance(node, ast.filter.StartsWith):
+            return self._starts_with(type_, node, head)
+        if isinstance(node, ast.filter.EndsWith):
+            return self._ends_with(type_, node, head)
+        if isinstance(node, ast.filter.LessThan):
+            return self._less_than(type_, node, head)
+        if isinstance(node, ast.filter.GreaterThan):
+            return self._greater_than(type_, node, head)
+        # invalid node
+        raise errors.BackendError(f'expected filter expression, found {node}')
+
+    def _parse_predicate_expression(
+            self,
+            type_: bsc.Vertex,
+            node: ast.filter.PredicateExpression
+            ) -> typing.Tuple[str, bsc.Vertex]:
+        """Route *node* to the handler of the respective PredicateExpression subclass."""
+        if isinstance(node, ast.filter.Predicate):
+            return self._predicate(type_, node)
+        if isinstance(node, ast.filter.OneOf):
+            return self._one_of(type_, node)
+        # invalid node
+        raise errors.BackendError(f'expected predicate expression, found {node}')
+
+    def _one_of(self, node_type: bsc.Vertex, node: ast.filter.OneOf) -> typing.Tuple[str, bsc.Vertex]:
+        """
+        """
+        if not isinstance(node_type, bsc.Node):
+            raise errors.BackendError(f'expected Node, found {node_type}')
+        # walk through predicates
+        suburi, rng = set(), None
+        for pred in node: # OneOf guarantees at least one expression
+            puri, subrng = self._parse_predicate_expression(node_type, pred)
+            # track predicate uris
+            suburi.add(puri)
+            # check for more generic range
+            if rng is None or subrng > rng:
+                rng = subrng
+            # check range consistency
+            if not subrng <= rng and not subrng >= rng:
+                raise errors.ConsistencyError(f'ranges {subrng} and {rng} are not related')
+        # return joint predicate expression and next range
+        # OneOf guarantees at least one expression, rng is always a bsc.Vertex.
+        # mypy does not realize this, hence we ignore the warning.
+        return '|'.join(suburi), rng # type: ignore [return-value]
+
+    def _predicate(self, node_type: bsc.Vertex, node: ast.filter.Predicate) -> typing.Tuple[str, bsc.Vertex]:
+        """
+        """
+        # check node_type
+        if not isinstance(node_type, bsc.Node):
+            raise errors.BackendError(f'expected Node, found {node_type}')
+        # fetch predicate and its uri
+        puri = node.predicate
+        # get and check predicate, domain, and range
+        if not self.schema.has_predicate(puri):
+            raise errors.ConsistencyError(f'predicate {puri} is not in the schema')
+        pred = self.schema.predicate(puri)
+        if not isinstance(pred.range, (bsc.Node, bsc.Literal)):
+            raise errors.BackendError(f'the range of predicate {pred} is undefined')
+        dom, rng = pred.domain, pred.range
+        # encapsulate predicate uri
+        uri_str = f'<{puri}>'
+        # apply reverse flag
+        if node.reverse:
+            uri_str = '^' + uri_str
+            dom, rng = rng, dom # type: ignore [assignment] # variable re-use confuses mypy
+        # check path consistency
+        if not node_type <= dom:
+            raise errors.ConsistencyError(f'expected type {dom} or subtype thereof, found {node_type}')
+        # return predicate URI and next node type
+        return uri_str, rng
+
+    def _any(self, node_type: bsc.Vertex, node: ast.filter.Any, head: str) -> str:
+        """
+        """
+        if not isinstance(node_type, bsc.Node):
+            raise errors.BackendError(f'expected Node, found {node_type}')
+        # parse predicate
+        pred, next_type = self._parse_predicate_expression(node_type, node.predicate)
+        # parse expression
+        nexthead = next(self.ngen)
+        expr = self._parse_filter_expression(next_type, node.expr, nexthead)
+        # combine results
+        return f'{head} {pred} {nexthead} . {expr}'
+
+    def _all(self, node_type: bsc.Vertex, node: ast.filter.All, head: str) -> str:
+        """
+        """
+        # NOTE: All(P, E) := Not(Any(P, Not(E))) and EXISTS(P, ?)
+        if not isinstance(node_type, bsc.Node):
+            raise errors.BackendError(f'expected Node, found {node_type}')
+        # parse rewritten ast
+        expr = self._parse_filter_expression(node_type,
+            ast.filter.Not(
+                ast.filter.Any(node.predicate,
+                    ast.filter.Not(node.expr))), head)
+        # parse predicate for existence constraint
+        pred, _ = self._parse_predicate_expression(node_type, node.predicate)
+        temphead = next(self.ngen)
+        # return existence and rewritten expression
+        return f'FILTER EXISTS {{ {head} {pred} {temphead} }} . ' + expr
+
+    def _and(self, node_type: bsc.Vertex, node: ast.filter.And, head: str) -> str:
+        """
+        """
+        sub = [self._parse_filter_expression(node_type, expr, head) for expr in node]
+        return ' . '.join(sub)
+
+    def _or(self, node_type: bsc.Vertex, node: ast.filter.Or, head: str) -> str:
+        """
+        """
+        # potential special case optimization:
+        # * ast: Or(Equals('foo'), Equals('bar'), ...)
+        # * query: VALUES ?head { "value1"^^<...> "value2"^^<...> "value3"^<...> ... }
+        sub = [self._parse_filter_expression(node_type, expr, head) for expr in node]
+        sub = ['{' + expr + '}' for expr in sub]
+        return ' UNION '.join(sub)
+
+    def _not(self, node_type: bsc.Vertex, node: ast.filter.Not, head: str) -> str:
+        """
+        """
+        expr = self._parse_filter_expression(node_type, node.expr, head)
+        if isinstance(node_type, bsc.Literal):
+            return f'MINUS {{ {expr} }}'
+        # NOTE: for bsc.Node types, we must include at least one expression in the body of MINUS,
+        # otherwise the connection between the context and body of MINUS is lost.
+        # The simplest (and non-interfering) choice is a type statement.
+        return f'MINUS {{ {head} <{ns.rdf.type}>/<{ns.rdfs.subClassOf}>* <{node_type.uri}> . {expr} }}'
+
+    def _has(self, node_type: bsc.Vertex, node: ast.filter.Has, head: str) -> str:
+        """
+        """
+        if not isinstance(node_type, bsc.Node):
+            raise errors.BackendError(f'expected Node, found {node_type}')
+        # parse predicate
+        pred, _ = self._parse_predicate_expression(node_type, node.predicate)
+        # get new heads
+        inner = next(self.ngen)
+        outer = next(self.ngen)
+        # predicate count expression (fetch number of predicates at *head*)
+        num_preds = f'{{ SELECT (COUNT(distinct {inner}) as {outer}) WHERE {{ {head} {pred} {inner} }} }}'
+        # count expression
+        count_bounds = self._parse_filter_expression(self.schema.literal(ns.xsd.integer), node.count, outer)
+        # combine
+        return num_preds + ' . ' + count_bounds
+
+    def _distance(self, node_type: bsc.Vertex, node: ast.filter.Distance, head: str) -> str:
+        """
+        """
+        if not isinstance(node_type, bsc.Feature):
+            raise errors.BackendError(f'expected Feature, found {node_type}')
+        if len(node.reference) != node_type.dimension:
+            raise errors.ConsistencyError(
+                f'reference has dimension {len(node.reference)}, expected {node_type.dimension}')
+        # get distance metric
+        dist = DISTANCE_FU[node_type.distance]
+        # get operator
+        cmp = operator.lt if node.strict else operator.le
+        # get candidate values
+        candidates = {
+            f'"{cand}"^^<{node_type.uri}>'
+            for cand
+            in self.graph.objects()
+            if isinstance(cand, rdflib.Literal)
+            and cand.datatype == rdflib.URIRef(node_type.uri)
+            and cmp(dist(cand.value, node.reference), node.threshold)
+            }
+        # combine candidate values
+        values = ' '.join(candidates) if len(candidates) else f'"impossible value"^^<{ns.xsd.string}>'
+        # return sparql fragment
+        return f'VALUES {head} {{ {values} }}'
+
+    def _is(self, node_type: bsc.Vertex, node: ast.filter.Is, head: str) -> str:
+        """
+        """
+        if not isinstance(node_type, bsc.Node):
+            raise errors.BackendError(f'expected Node, found {node_type}')
+        return f'VALUES {head} {{ <{URI(node.value)}> }}'
+
+    def _equals(self, node_type: bsc.Vertex, node: ast.filter.Equals, head: str) -> str:
+        """
+        """
+        if not isinstance(node_type, bsc.Literal):
+            raise errors.BackendError(f'expected Literal, found {node}')
+        return f'VALUES {head} {{ "{node.value}"^^<{node_type.uri}> }}'
+
+    def _substring(self, node_type: bsc.Vertex, node: ast.filter.Substring, head: str) -> str:
+        """
+        """
+        if not isinstance(node_type, bsc.Literal):
+            raise errors.BackendError(f'expected Literal, found {node_type}')
+        return f'FILTER contains(str({head}), "{node.value}")'
+
+    def _starts_with(self, node_type: bsc.Vertex, node: ast.filter.StartsWith, head: str) -> str:
+        """
+        """
+        if not isinstance(node_type, bsc.Literal):
+            raise errors.BackendError(f'expected Literal, found {node_type}')
+        return f'FILTER strstarts(str({head}), "{node.value}")'
+
+    def _ends_with(self, node_type: bsc.Vertex, node: ast.filter.EndsWith, head: str) -> str:
+        """
+        """
+        if not isinstance(node_type, bsc.Literal):
+            raise errors.BackendError(f'expected Literal, found {node_type}')
+        return f'FILTER strends(str({head}), "{node.value}")'
+
+    def _less_than(self, node_type: bsc.Vertex, node: ast.filter.LessThan, head: str) -> str:
+        """
+        """
+        if not isinstance(node_type, bsc.Literal):
+            raise errors.BackendError(f'expected Literal, found {node_type}')
+        equality = '=' if not node.strict else ''
+        return f'FILTER ({head} <{equality} {float(node.threshold)})'
+
+    def _greater_than(self, node_type: bsc.Vertex, node: ast.filter.GreaterThan, head: str) -> str:
+        """
+        """
+        if not isinstance(node_type, bsc.Literal):
+            raise errors.BackendError(f'expected Literal, found {node_type}')
+        equality = '=' if not node.strict else ''
+        return f'FILTER ({head} >{equality} {float(node.threshold)})'
+
+## EOF ##
diff --git a/bsfs/triple_store/sparql.py b/bsfs/triple_store/sparql/sparql.py
index 7516dff..99e67d6 100644
--- a/bsfs/triple_store/sparql.py
+++ b/bsfs/triple_store/sparql/sparql.py
@@ -1,20 +1,23 @@
-"""
 
-Part of the BlackStar filesystem (bsfs) module.
-A copy of the license is provided with the project.
-Author: Matthias Baumgartner, 2022
-"""
 # imports
+import base64
 import itertools
 import typing
+
+# external imports
 import rdflib
 
 # bsfs imports
 from bsfs import schema as bsc
+from bsfs.namespace import ns
+from bsfs.query import ast
 from bsfs.utils import errors, URI
 
 # inner-module imports
-from . import base
+from . import parse_fetch
+from . import parse_filter
+from .. import base
+from .distance import DISTANCE_FU
 
 
 # exports
@@ -25,6 +28,8 @@ __all__: typing.Sequence[str] = (
 
 ## code ##
 
+rdflib.term.bind(ns.bsl.BinaryBlob, bytes, constructor=base64.b64decode)
+
 class _Transaction():
     """Lightweight rdflib transactions for in-memory databases."""
 
@@ -85,11 +90,19 @@ class SparqlStore(base.TripleStoreBase):
     # The local schema.
     _schema: bsc.Schema
 
+    # Filter parser
+    _filter_parser: parse_filter.Filter
+
+    # Fetch parser
+    _fetch_parser: parse_fetch.Fetch
+
     def __init__(self):
         super().__init__(None)
         self._graph = rdflib.Graph()
         self._transaction = _Transaction(self._graph)
-        self._schema = bsc.Schema.Empty()
+        self._schema = bsc.Schema(literals={bsc.ROOT_NUMBER.child(ns.xsd.integer)})
+        self._filter_parser = parse_filter.Filter(self._graph, self._schema)
+        self._fetch_parser = parse_fetch.Fetch(self._schema)
 
     # NOTE: mypy and pylint complain about the **kwargs not being listed (contrasting super)
     # However, not having it here is clearer since it's explicit that there are no arguments.
@@ -115,6 +128,16 @@ class SparqlStore(base.TripleStoreBase):
         # check compatibility: No contradicting definitions
         if not self.schema.consistent_with(schema):
             raise errors.ConsistencyError(f'{schema} is inconsistent with {self.schema}')
+        # check distance functions of features
+        invalid = {
+            (cand.uri, cand.distance)
+            for cand
+            in schema.literals()
+            if isinstance(cand, bsc.Feature) and cand.distance not in DISTANCE_FU}
+        if len(invalid) > 0:
+            cand, dist = zip(*invalid)
+            raise errors.UnsupportedError(
+                f'unknown distance function {",".join(dist)} in feature {", ".join(cand)}')
 
         # commit the current transaction
         self.commit()
@@ -126,10 +149,17 @@ class SparqlStore(base.TripleStoreBase):
         # get deleted classes
         sub = self.schema - schema
 
-        # remove predicate instances
         for pred in sub.predicates:
+            # remove predicate instances
             for src, trg in self._graph.subject_objects(rdflib.URIRef(pred.uri)):
                 self._transaction.remove((src, rdflib.URIRef(pred.uri), trg))
+            # remove predicate definition
+            if pred.parent is not None: # NOTE: there shouldn't be any predicate w/o parent
+                self._transaction.remove((
+                    rdflib.URIRef(pred.uri),
+                    rdflib.RDFS.subClassOf,
+                    rdflib.URIRef(pred.parent.uri),
+                    ))
 
         # remove node instances
         for node in sub.nodes:
@@ -143,15 +173,82 @@ class SparqlStore(base.TripleStoreBase):
                     self._transaction.remove((inst, pred, trg))
                 # remove instance
                 self._transaction.remove((inst, rdflib.RDF.type, rdflib.URIRef(node.uri)))
-
-        # NOTE: Nothing to do for literals
+            # remove node definition
+            if node.parent is not None: # NOTE: there shouldn't be any node w/o parent
+                self._transaction.remove((
+                    rdflib.URIRef(node.uri),
+                    rdflib.RDFS.subClassOf,
+                    rdflib.URIRef(node.parent.uri),
+                    ))
+
+        for lit in sub.literals:
+            # remove literal definition
+            if lit.parent is not None: # NOTE: there shouldn't be any literal w/o parent
+                self._transaction.remove((
+                    rdflib.URIRef(lit.uri),
+                    rdflib.RDFS.subClassOf,
+                    rdflib.URIRef(lit.parent.uri),
+                    ))
+
+        # add predicate, node, and literal hierarchies to the graph
+        for itm in itertools.chain(schema.predicates(), schema.nodes(), schema.literals()):
+            if itm.parent is not None:
+                self._transaction.add((rdflib.URIRef(itm.uri), rdflib.RDFS.subClassOf, rdflib.URIRef(itm.parent.uri)))
 
         # commit instance changes
         self.commit()
 
         # migrate schema
         self._schema = schema
+        self._filter_parser.schema = schema
+        self._fetch_parser.schema = schema
 
+    def fetch(
+            self,
+            node_type: bsc.Node,
+            filter: ast.filter.FilterExpression, # pylint: disable=redefined-builtin
+            fetch: ast.fetch.FetchExpression,
+            ) -> typing.Iterator[typing.Tuple[URI, str, typing.Any]]:
+        if node_type not in self.schema.nodes():
+            raise errors.ConsistencyError(f'{node_type} is not defined in the schema')
+        if not isinstance(filter, ast.filter.FilterExpression):
+            raise TypeError(filter)
+        if not isinstance(fetch, ast.fetch.FetchExpression):
+            raise TypeError(fetch)
+        # compose a query from fetch and filter ast
+        query = self._filter_parser(node_type, filter)
+        query += self._fetch_parser(node_type, fetch)
+        # run query
+        emitted = set()
+        for result in query(self._graph):
+            guid = URI(result[0])
+            for name, raw in zip(query.names, result[1:]):
+                if raw is None: # undefined value
+                    continue
+                if isinstance(raw, rdflib.Literal):
+                    value = raw.value
+                else:
+                    value = URI(raw)
+                # emit triple
+                triple = (guid, name, value)
+                if triple not in emitted: # FIXME: needs a better solution!
+                    emitted.add(triple)
+                    yield guid, name, value
+
+    def get(
+            self,
+            node_type: bsc.Node,
+            filter: typing.Optional[ast.filter.FilterExpression] = None, # pylint: disable=redefined-builtin
+            ) -> typing.Iterator[URI]:
+        if node_type not in self.schema.nodes():
+            raise errors.ConsistencyError(f'{node_type} is not defined in the schema')
+        if filter is not None and not isinstance(filter, ast.filter.FilterExpression):
+            raise TypeError(filter)
+        # compose query
+        query = self._filter_parser(node_type, filter)
+        # run query
+        for guid, in query(self._graph):
+            yield URI(guid)
 
     def _has_type(self, subject: URI, node_type: bsc.Node) -> bool:
         """Return True if *subject* is a node of class *node_type* or a subclass thereof."""
@@ -187,7 +284,7 @@ class SparqlStore(base.TripleStoreBase):
             raise errors.ConsistencyError(f'{node_type} is not defined in the schema')
         # check and create guids
         for guid in guids:
-            subject = rdflib.URIRef(guid)
+            subject = rdflib.URIRef(URI(guid))
             # check node existence
             if (subject, rdflib.RDF.type, None) in self._graph:
                 # FIXME: node exists and may have a different type! ignore? raise? report?
@@ -226,7 +323,7 @@ class SparqlStore(base.TripleStoreBase):
                 raise errors.InstanceError(inconsistent)
         # check guids
         # FIXME: Fail or skip inexistent nodes?
-        guids = set(guids)
+        guids = {URI(guid) for guid in guids}
         inconsistent = {guid for guid in guids if not self._has_type(guid, node_type)}
         if len(inconsistent) > 0:
             raise errors.InstanceError(inconsistent)
@@ -237,7 +334,11 @@ class SparqlStore(base.TripleStoreBase):
             guid = rdflib.URIRef(guid)
             # convert value
             if isinstance(predicate.range, bsc.Literal):
-                value = rdflib.Literal(value, datatype=rdflib.URIRef(predicate.range.uri))
+                dtype = rdflib.URIRef(predicate.range.uri)
+                if predicate.range <= self.schema.literal(ns.bsl.BinaryBlob):
+                    dtype = rdflib.URIRef(ns.bsl.BinaryBlob)
+                    value = base64.b64encode(value)
+                value = rdflib.Literal(value, datatype=dtype)
             elif isinstance(predicate.range, bsc.Node):
                 value = rdflib.URIRef(value)
             else:
diff --git a/bsfs/triple_store/sparql/utils.py b/bsfs/triple_store/sparql/utils.py
new file mode 100644
index 0000000..38062c2
--- /dev/null
+++ b/bsfs/triple_store/sparql/utils.py
@@ -0,0 +1,137 @@
+
+# standard imports
+import typing
+
+# external imports
+import rdflib
+
+# bsfs imports
+from bsfs.namespace import ns
+from bsfs.utils import typename
+
+# exports
+__all__: typing.Sequence[str] = (
+    'GenHopName',
+    'Query',
+    )
+
+
+## code ##
+
+class GenHopName():
+    """Generator that produces a new unique symbol name with each iteration."""
+
+    # Symbol name prefix.
+    prefix: str
+
+    # Current counter.
+    curr: int
+
+    def __init__(self, prefix: str = '?hop', start: int = 0):
+        self.prefix = prefix
+        self.curr = start - 1
+
+    def __next__(self):
+        """Generate and return the next unique name."""
+        self.curr += 1
+        return self.prefix + str(self.curr)
+
+
+class Query():
+    """Hold, manage, and complete partial Sparql queries."""
+
+    # root node type URI.
+    root_type: str
+
+    # root node variable name.
+    root_head: str
+
+    # (head, name) tuples (w/o root)
+    select: typing.Tuple[typing.Tuple[str, str], ...]
+
+    # where statements.
+    where: str
+
+    def __init__(
+            self,
+            root_type: str,
+            root_head: str = '?ent',
+            select: typing.Optional[typing.Iterable[typing.Tuple[str, str]]] = None,
+            where: typing.Optional[str] = None,
+            ):
+        # check arguments
+        if select is None:
+            select = []
+        if where is None:
+            where = ''
+        # set members
+        self.root_type = root_type
+        self.root_head = root_head
+        self.select = tuple(select) # tuple ensures presistent order
+        self.where = where.strip()
+
+    def __str__(self) -> str:
+        return self.query
+
+    def __repr__(self) -> str:
+        return f'{typename(self)}({self.root_type}, {self.root_head}, {self.select}, {self.where})'
+
+    def __eq__(self, other: typing.Any) -> bool:
+        return isinstance(other, type(self)) \
+           and self.root_type == other.root_type \
+           and self.root_head == other.root_head \
+           and self.select == other.select \
+           and self.where == other.where
+
+    def __hash__(self) -> int:
+        return hash((type(self), self.root_type, self.root_head, self.select, self.where))
+
+    def __add__(self, other: typing.Any) -> 'Query':
+        # check other's type
+        if not isinstance(other, type(self)):
+            return NotImplemented
+        # check query compatibility
+        if not self.root_type == other.root_type:
+            raise ValueError(other)
+        if not self.root_head == other.root_head:
+            raise ValueError(other)
+        # combine selections
+        select = self.select + other.select
+        # combine conditions
+        conds = []
+        if self.where != '':
+            conds.append(self.where)
+        if other.where != '':
+            conds.append(other.where)
+        where = ' . '.join(conds)
+        # return new query
+        return Query(
+            root_type=self.root_type,
+            root_head=self.root_head,
+            select=select,
+            where=where,
+            )
+
+    @property
+    def names(self) -> typing.Tuple[str, ...]:
+        """Return a tuple of selected variable names, excluding the root."""
+        return tuple(name for _, name in self.select)
+
+    @property
+    def query(self) -> str:
+        """Return an executable sparql query."""
+        select = ' '.join(f'({head} as ?{name})' for head, name in self.select)
+        return f'''
+            SELECT DISTINCT {self.root_head} {select}
+            WHERE {{
+                {self.root_head} <{ns.rdf.type}>/<{ns.rdfs.subClassOf}>* <{self.root_type}> .
+                {self.where}
+            }}
+            ORDER BY str({self.root_head})
+            '''
+
+    def __call__(self, graph: rdflib.Graph) -> rdflib.query.Result:
+        """Execute the query on a *graph* and return the query result."""
+        return graph.query(self.query)
+
+## EOF ##
diff --git a/bsfs/utils/__init__.py b/bsfs/utils/__init__.py
index 94680ee..d497645 100644
--- a/bsfs/utils/__init__.py
+++ b/bsfs/utils/__init__.py
@@ -1,15 +1,10 @@
-"""
 
-Part of the BlackStar filesystem (bsfs) module.
-A copy of the license is provided with the project.
-Author: Matthias Baumgartner, 2022
-"""
 # imports
 import typing
 
 # inner-module imports
 from . import errors
-from .commons import typename
+from .commons import typename, normalize_args
 from .uri import URI
 from .uuid import UUID, UCID
 
@@ -19,6 +14,7 @@ __all__ : typing.Sequence[str] = (
     'URI',
     'UUID',
     'errors',
+    'normalize_args',
     'typename',
     )
 
diff --git a/bsfs/utils/commons.py b/bsfs/utils/commons.py
index bad2fe0..a7092ae 100644
--- a/bsfs/utils/commons.py
+++ b/bsfs/utils/commons.py
@@ -1,14 +1,11 @@
-"""
 
-Part of the BlackStar filesystem (bsfs) module.
-A copy of the license is provided with the project.
-Author: Matthias Baumgartner, 2022
-"""
 # imports
+from collections import abc
 import typing
 
 # exports
 __all__: typing.Sequence[str] = (
+    'normalize_args',
     'typename',
     )
 
@@ -19,5 +16,37 @@ def typename(obj) -> str:
     """Return the type name of *obj*."""
     return type(obj).__name__
 
+# argument type in `normalize_args`.
+ArgType = typing.TypeVar('ArgType') # pylint: disable=invalid-name # type vars don't follow the usual convention
+
+def normalize_args(
+        *args: typing.Union[ArgType, typing.Iterable[ArgType], typing.Iterator[ArgType]]
+        ) -> typing.Tuple[ArgType, ...]:
+    """Arguments to a function can be passed as individual arguments, list-like
+    structures, or iterables. This function processes any of these styles and
+    returns a tuple of the respective items. Typically used within a function
+    provide a flexible interface but sill have parameters in a normalized form.
+
+    Examples:
+
+    >>> normalize_args(0,1,2)
+    (1,2,3)
+    >>> normalize_args([0,1,2])
+    (1,2,3)
+    >>> normalize_args(range(3))
+    (1,2,3)
+
+    """
+    if len(args) == 0: # foo()
+        return tuple()
+    if len(args) > 1: # foo(0, 1, 2)
+        return tuple(args) # type: ignore [arg-type] # we assume that argument styles (arg vs. iterable) are not mixed.
+    if isinstance(args[0], abc.Iterator): # foo(iter([0,1,2]))
+        return tuple(args[0])
+    if isinstance(args[0], abc.Iterable) and not isinstance(args[0], str): # foo([0, 1, 2])
+        return tuple(args[0])
+    # foo(0)
+    return (args[0], ) # type: ignore [return-value] # if args[0] is a str, we assume that ArgType was str.
+
 
 ## EOF ##
diff --git a/bsfs/utils/errors.py b/bsfs/utils/errors.py
index c5e8e16..b82e6e2 100644
--- a/bsfs/utils/errors.py
+++ b/bsfs/utils/errors.py
@@ -1,9 +1,4 @@
-"""
 
-Part of the BlackStar filesystem (bsfs) module.
-A copy of the license is provided with the project.
-Author: Matthias Baumgartner, 2022
-"""
 # imports
 import typing
 
@@ -38,4 +33,10 @@ class UnreachableError(ProgrammingError):
 class ConfigError(_BSFSError):
     """User config issue."""
 
+class BackendError(_BSFSError):
+    """Could not parse an AST structure."""
+
+class UnsupportedError(_BSFSError):
+    """Some requested functionality is not supported by an implementation."""
+
 ## EOF ##
diff --git a/bsfs/utils/uri.py b/bsfs/utils/uri.py
index 84854a4..5755a6e 100644
--- a/bsfs/utils/uri.py
+++ b/bsfs/utils/uri.py
@@ -1,14 +1,11 @@
-"""
 
-Part of the BlackStar filesystem (bsfs) module.
-A copy of the license is provided with the project.
-Author: Matthias Baumgartner, 2022
-"""
 # imports
 import re
 import typing
 
 # constants
+RX_CHARS = re.compile(r'[<>" {}|\\^]')
+
 RX_URI = re.compile(r'''
     ^
     (?:(?P<scheme>[^:/?#]+):)?      # scheme, ://-delimited
@@ -82,6 +79,9 @@ class URI(str):
         no claim about the validity of an URI!
 
         """
+        # check characters
+        if RX_CHARS.search(query) is not None:
+            return False
         # check uri
         parts = RX_URI.match(query)
         if parts is not None:
@@ -232,9 +232,6 @@ class URI(str):
 
     # overload formatting methods
 
-    def format(self, *args, **kwargs) -> 'URI':
-        return URI(super().format(*args, **kwargs))
-
     def __mod__(self, *args) -> 'URI':
         return URI(super().__mod__(*args))
 
diff --git a/bsfs/utils/uuid.py b/bsfs/utils/uuid.py
index 6366b18..ad7fc1c 100644
--- a/bsfs/utils/uuid.py
+++ b/bsfs/utils/uuid.py
@@ -1,12 +1,9 @@
-"""
 
-Part of the BlackStar filesystem (bsfs) module.
-A copy of the license is provided with the project.
-Author: Matthias Baumgartner, 2022
-"""
 # imports
 from collections import abc
 import hashlib
+import io
+import json
 import os
 import platform
 import random
@@ -105,4 +102,21 @@ class UCID():
         with open(path, 'rb') as ifile:
             return HASH(ifile.read()).hexdigest()
 
+    @staticmethod
+    def from_buffer(buffer: io.IOBase) -> str:
+        """Read the content from a buffer."""
+        if isinstance(buffer, io.TextIOBase):
+            return HASH(buffer.read().encode('utf-8', errors='ignore')).hexdigest()
+        return HASH(buffer.read()).hexdigest()
+
+    @staticmethod
+    def from_bytes(content: bytes) -> str:
+        """Get the content from as bytes."""
+        return HASH(content).hexdigest()
+
+    @staticmethod
+    def from_dict(content: dict) -> str:
+        """Get the content from a dict."""
+        return HASH(json.dumps(content).encode('ascii', 'ignore')).hexdigest()
+
 ## EOF ##
diff --git a/doc/Makefile b/doc/Makefile
new file mode 100644
index 0000000..d0c3cbf
--- /dev/null
+++ b/doc/Makefile
@@ -0,0 +1,20 @@
+# Minimal makefile for Sphinx documentation
+#
+
+# You can set these variables from the command line, and also
+# from the environment for the first two.
+SPHINXOPTS    ?=
+SPHINXBUILD   ?= sphinx-build
+SOURCEDIR     = source
+BUILDDIR      = build
+
+# Put it first so that "make" without argument is like "make help".
+help:
+	@$(SPHINXBUILD) -M help "$(SOURCEDIR)" "$(BUILDDIR)" $(SPHINXOPTS) $(O)
+
+.PHONY: help Makefile
+
+# Catch-all target: route all unknown targets to Sphinx using the new
+# "make mode" option.  $(O) is meant as a shortcut for $(SPHINXOPTS).
+%: Makefile
+	@$(SPHINXBUILD) -M $@ "$(SOURCEDIR)" "$(BUILDDIR)" $(SPHINXOPTS) $(O)
diff --git a/doc/make.bat b/doc/make.bat
new file mode 100644
index 0000000..747ffb7
--- /dev/null
+++ b/doc/make.bat
@@ -0,0 +1,35 @@
+@ECHO OFF

+

+pushd %~dp0

+

+REM Command file for Sphinx documentation

+

+if "%SPHINXBUILD%" == "" (

+	set SPHINXBUILD=sphinx-build

+)

+set SOURCEDIR=source

+set BUILDDIR=build

+

+%SPHINXBUILD% >NUL 2>NUL

+if errorlevel 9009 (

+	echo.

+	echo.The 'sphinx-build' command was not found. Make sure you have Sphinx

+	echo.installed, then set the SPHINXBUILD environment variable to point

+	echo.to the full path of the 'sphinx-build' executable. Alternatively you

+	echo.may add the Sphinx directory to PATH.

+	echo.

+	echo.If you don't have Sphinx installed, grab it from

+	echo.https://www.sphinx-doc.org/

+	exit /b 1

+)

+

+if "%1" == "" goto help

+

+%SPHINXBUILD% -M %1 %SOURCEDIR% %BUILDDIR% %SPHINXOPTS% %O%

+goto end

+

+:help

+%SPHINXBUILD% -M help %SOURCEDIR% %BUILDDIR% %SPHINXOPTS% %O%

+

+:end

+popd

diff --git a/doc/source/_static/arch_dark.png b/doc/source/_static/arch_dark.png
new file mode 100644
index 0000000..b5ea1b3
--- /dev/null
+++ b/doc/source/_static/arch_dark.png
diff --git a/doc/source/_static/arch_dark.svg b/doc/source/_static/arch_dark.svg
new file mode 100644
index 0000000..22de237
--- /dev/null
+++ b/doc/source/_static/arch_dark.svg
@@ -0,0 +1,500 @@
+<?xml version="1.0" encoding="UTF-8" standalone="no"?>
+<!-- Created with Inkscape (http://www.inkscape.org/) -->
+
+<svg
+   width="226.45284mm"
+   height="347.16772mm"
+   viewBox="0 0 226.45283 347.16772"
+   version="1.1"
+   id="svg8"
+   inkscape:version="1.1.2 (0a00cf5339, 2022-02-04)"
+   sodipodi:docname="architecture.svg"
+   inkscape:export-filename="/home/matthias/projects/black_star/modules/bsfs/doc/source/_static/architecture.png"
+   inkscape:export-xdpi="36.581741"
+   inkscape:export-ydpi="36.581741"
+   xmlns:inkscape="http://www.inkscape.org/namespaces/inkscape"
+   xmlns:sodipodi="http://sodipodi.sourceforge.net/DTD/sodipodi-0.dtd"
+   xmlns="http://www.w3.org/2000/svg"
+   xmlns:svg="http://www.w3.org/2000/svg"
+   xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
+   xmlns:cc="http://creativecommons.org/ns#"
+   xmlns:dc="http://purl.org/dc/elements/1.1/">
+  <defs
+     id="defs2">
+    <marker
+       style="overflow:visible"
+       id="marker5768"
+       refX="0.0"
+       refY="0.0"
+       orient="auto"
+       inkscape:stockid="Arrow2Mstart"
+       inkscape:isstock="true">
+      <path
+         transform="scale(0.6) translate(0,0)"
+         d="M 8.7185878,4.0337352 L -2.2072895,0.016013256 L 8.7185884,-4.0017078 C 6.9730900,-1.6296469 6.9831476,1.6157441 8.7185878,4.0337352 z "
+         style="stroke:context-stroke;fill-rule:evenodd;fill:context-stroke;stroke-width:0.62500000;stroke-linejoin:round"
+         id="path5499" />
+    </marker>
+    <marker
+       style="overflow:visible;"
+       id="Arrow2Mend"
+       refX="0.0"
+       refY="0.0"
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+       inkscape:stockid="Arrow2Mend"
+       inkscape:isstock="true">
+      <path
+         transform="scale(0.6) rotate(180) translate(0,0)"
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+         id="path5502" />
+    </marker>
+    <marker
+       style="overflow:visible;"
+       id="Arrow1Send"
+       refX="0.0"
+       refY="0.0"
+       orient="auto"
+       inkscape:stockid="Arrow1Send"
+       inkscape:isstock="true">
+      <path
+         transform="scale(0.2) rotate(180) translate(6,0)"
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+    <marker
+       style="overflow:visible;"
+       id="Arrow1Mend"
+       refX="0.0"
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+       inkscape:stockid="Arrow1Mend"
+       inkscape:isstock="true">
+      <path
+         transform="scale(0.4) rotate(180) translate(10,0)"
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+         id="path5484" />
+    </marker>
+    <marker
+       style="overflow:visible"
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diff --git a/doc/source/architecture.rst b/doc/source/architecture.rst
new file mode 100644
index 0000000..4cca49a
--- /dev/null
+++ b/doc/source/architecture.rst
@@ -0,0 +1,87 @@
+
+Architecture
+============
+
+The BSFS stack can be coarsely divided into four parts (see the image below).
+
+* Envelope: Essentials and utils used throughout the whole codebase.
+* Front: End-user applications and APIs.
+* Center: The core interfaces and functionality.
+* Back: The triple store backends.
+
+Details of these components are given in the sections below.
+
+
+.. image:: _static/arch_light.png
+   :class: only-light
+
+.. image:: _static/arch_dark.png
+   :class: only-dark
+
+
+Envelope
+--------
+
+Most notably, the envelope covers the :class:`Schema <bsfs.schema.schema.Schema>` and the :mod:`Query syntax trees (AST) <bsfs.query.ast>`.
+Both of them essential for all parts of the BSFS stack.
+For example, the schema is specified by the user via the :func:`Migrate <bsfs.apps.migrate.main>` command, checked and extended by the :class:`Graph <bsfs.graph.graph.Graph>`, and ultimately stored by a :class:`Triple Store backend <bsfs.triple_store.base.TripleStoreBase>`.
+Similarly, the Query AST may be provided by a caller and is translated to a database query by a backend.
+In addition, the envelope also contains some classes to handle URIs: 
+:class:`URI <bsfs.utils.uri.URI>` defines the URI base class,
+:class:`Namespace <bsfs.namespace.Namespace>` provides shortcuts to generate URIs, and
+:mod:`UUID <bsfs.utils.uuid>` is used to generate unique URIs.
+
+
+Front
+-----
+
+The front consists of exposed interfaces such as end-user applications or APIs,
+and all utils needed to offer this functionality.
+See :mod:`bsfs.apps` and :mod:`bsfs.front`.
+
+
+Center
+------
+
+The heart of BSFS is grouped around the :mod:`bsfs.graph` module.
+These classes provide the interface to navigate and manipulate the file graph
+in a safe and programmer friendly manner.
+Some of them are indirectly exposed through the higher-level APIs.
+
+The two core design principles of BSFS are the focus on nodes and batch processing.
+They are realized in the the Graph and Nodes classes.
+The :class:`Graph class <bsfs.graph.graph.Graph>` manages the graph as a whole,
+and offers methods to get a specific set of Nodes.
+In turn, the :class:`Nodes class <bsfs.graph.nodes.Nodes>` represents such a set of nodes,
+and performs operations on the whole node set at once.
+Besides, the :mod:`bsfs.graph` module also comes with some syntactic sugar.
+
+Example::
+
+    # Open a file graph.
+    from bsfs import Open, ns
+    graph = Open(...)
+    # Get all nodes of type File.
+    nodes = graph.all(ns.bsfs.File)
+    # Set the author of all nodes at once.
+    nodes.set(ns.bse.author, 'Myself')
+    # Retrieve the author of all nodes at once.
+    set(nodes.get(ns.bse.author, node=False))
+    # Same as above, but shorter.
+    set(nodes.comment(node=False))
+
+
+Back
+----
+
+There are various graph databases (e.g., `RDFLib`_, `Blazegraph`_, `Titan`_, etc.)
+and it would be foolish to replicate the work that others have done.
+Instead, we use third-party stores that take care of how to store and manage the data.
+The :class:`Backend base class <bsfs.triple_store.base.TripleStoreBase>` defines the
+interface to integrate any such third-party store to BSFS.
+Besides storing the data, a triple store backend also need to track the current schema.
+
+
+.. _RDFLib: https://rdflib.readthedocs.io/en/stable/index.html
+.. _Blazegraph: https://blazegraph.com/
+.. _Titan: http://titan.thinkaurelius.com/
diff --git a/doc/source/concepts.rst b/doc/source/concepts.rst
new file mode 100644
index 0000000..9c2ed43
--- /dev/null
+++ b/doc/source/concepts.rst
@@ -0,0 +1,98 @@
+
+Core concepts
+=============
+
+In the following, we present a few core concepts that should help in understanding the BSFS operations and codebase.
+
+
+Graph storage
+-------------
+
+`RDF`_ describes a network or graph like the file graph as a set of
+*(subject, predicate, object)* triples.
+*Subject* is the identifier of the source node,
+*object* is the identifier of the target node (or a literal value),
+and *predicate* is the type of relation between the source node and the target.
+As suggested by `RDF`_, we use URIs to identify nodes and predicates.
+For example, a triple that assigns me as the author of a file could look like this::
+
+    <http://example.com/file#1234> <https://bsfs.io/schema/Entity#author> <http://example.com/me>
+
+Note that alternatively, the *object* could also be a literal value ("me")::
+    
+    <http://example.com/file#1234> <https://bsfs.io/schema/Entity#author> "me"
+
+There are a number of graph databases that support this or an analoguous paradigm,
+such as `RDFLib`_, `Blazegraph`_, `TypeDB`_, `Titan`_,
+and `many more <https://en.wikipedia.org/wiki/Graph_database#List_of_graph_databases>`_.
+BSFS uses such a third-party graph database to store its file graph.
+
+As usual in database systems,
+we have to distinguish schema data (that coverns the structure of the storage)
+from instance data (the actual database content).
+Similar to relational database systems,
+both kinds of data can be represented as triples,
+and subsequently stored within the same graph storage
+(although one might need to separate them logically).
+In BSFS, we employ an explicit schema (see next section) that is managed alongside the data.
+
+
+
+Schema
+------
+
+BSFS ensures consistency across multiple distributed client applications
+by maintaining an explicit schema that governs node types and predicates.
+Furthermore, exposing the schema allows client to run a number of compatibility and validity checks
+locally, and a graph database may use the schema to optimize its storage or operations.
+
+In BSFS, the schema is initially provided by the system administrator
+(usually in the `Turtle`_ format)
+and subsequently stored by the backend.
+The default schema defines three root types
+(``bsfs:Node``, ``bsfs:Predicate``, and ``bsfs:Literal``),
+and BSFS expects any node, literal, or predicate to be derived from these roots.
+
+For example, a new predicate can be defined like so::
+
+    # define some abbreviations
+    prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
+    prefix bsfs: <http://schema.bsfs.io/>
+    prefix bse: <http://schema.bsfs.io/Entity#>
+
+    # define a node type
+    bsfs:Entity rdfs:subClassOf bsfs:Node .
+
+    # define a literal type
+    xsd:string rdfs:subClassOf bsfs:Literal .
+
+    # define a predicate ("author of a node")
+    bse:author rdfs:subClassOf bsfs:Predicate ;
+        rdfs:domain bsfs:Entity ;
+        rdfs:range xsd:string .
+
+BSFS checks all requests and rejects queries or operations that violate the schema.
+
+
+Querying
+--------
+
+BSFS at its core is not much more than a translator from a user query into a graph database query.
+It operates directly on three abstract syntax trees (AST),
+to run fetch, search, or sort, queries respectively.
+By not using an existing query language,
+we avoid an unnecessary and possibly expensive parsing step.
+Some routines create an AST internally (e.g., :func:`bsfs.graph.graph.Graph.all`),
+others accept an user-defined AST (e.g., :func:`bsfs.graph.graph.Graph.get`).
+One way or another, the AST is validated against the schema,
+and access control conditions are added.
+
+
+.. _RDF: https://www.w3.org/RDF/
+.. _RDFLib: https://rdflib.readthedocs.io/en/stable/index.html
+.. _Blazegraph: https://blazegraph.com/
+.. _Titan: http://titan.thinkaurelius.com/
+.. _TypeDB: https://vaticle.com/
+.. _Turtle: https://www.w3.org/TR/turtle/
+
+
diff --git a/doc/source/conf.py b/doc/source/conf.py
new file mode 100644
index 0000000..6de4993
--- /dev/null
+++ b/doc/source/conf.py
@@ -0,0 +1,37 @@
+# Configuration file for the Sphinx documentation builder.
+#
+# For the full list of built-in configuration values, see the documentation:
+# https://www.sphinx-doc.org/en/master/usage/configuration.html
+
+# -- Project information -----------------------------------------------------
+# https://www.sphinx-doc.org/en/master/usage/configuration.html#project-information
+
+project = 'Black Star File System'
+copyright = '2023, Matthias Baumgartner'
+author = 'Matthias Baumgartner'
+release = '0.5'
+
+# -- General configuration ---------------------------------------------------
+# https://www.sphinx-doc.org/en/master/usage/configuration.html#general-configuration
+
+extensions = [
+    'sphinx_copybutton',
+    'sphinx.ext.autodoc',
+    ]
+
+templates_path = ['_templates']
+exclude_patterns = []
+
+
+
+# -- Options for HTML output -------------------------------------------------
+# https://www.sphinx-doc.org/en/master/usage/configuration.html#options-for-html-output
+
+html_theme = 'furo'
+html_static_path = ['_static']
+
+html_title = 'bsfs'
+html_theme_options = {
+    'announcement': '<em>This project is under heavy development and subject to rapid changes. Use at your own discretion.</em>',
+    }
+
diff --git a/doc/source/index.rst b/doc/source/index.rst
new file mode 100644
index 0000000..91d53f6
--- /dev/null
+++ b/doc/source/index.rst
@@ -0,0 +1,75 @@
+
+The Black Star File System
+==========================
+
+A file system has two roles: It has to specify how to write files to a medium, and it has to define how a user can access files.
+Most file systems focus on the first role and adopt the standard directory tree approach for the second role.
+It is of course necessary to solve the challenges of medium access, but we should not neglect the user's perspective.
+As a user, I mostly care about how how conveniently I can organize my data, and quickly I can access relevant information.
+The hierarchical approach is rather restrictive in this regard:
+You can only organize files in a directory tree [#f1]_, and search tasks often require third-party tools like `find`_ or `locate`_.
+
+Tagging file systems proposed an alternative file organization model.
+Instead of a placing files in directories, they assign one or more (user-defined) tags to each file.
+This increases the flexibility over a hierarchical data model,
+because you can group any combination of files, and each file can be a part of various groups.
+Semantic file systems push this idea one step further by trying to understand
+the data they're dealing with.
+For example, files can be grouped by their data type (documents), file format (odt),
+author (yourself), topic (information management), etc.
+The benefit for the user is that they can browse their files by association rather than by location --- similar to how we nagivate the Web.
+
+Clearly, the hierarchical approach is insufficient to organize this variety of information.
+Instead, we need a network of files,
+where they can be connected to each other, their properties, or to auxiliary nodes
+(such as tags, collections, etc.) under a given relationship.
+We call this the file graph.
+With the *Black Star File System (BSFS)*, you can store, manage, and query such a file graph.
+
+..
+    TODO: Clarify
+    * Different relationships
+    * Properties and auxiliary nodes
+
+    TODO: File graph image
+    TODO: SFS/TFS references
+
+    TODO: BSFS features
+    Within BSFS, you can store the file content, file metadata,
+    and content-derived information (e.g., features) alike.
+
+    Within the file graph, we link files directly,
+    through properties, or through intermediate nodes.
+
+The Black Star File System is designed with three query patterns in mind:
+navigation, search, and browsing.
+
+The **navigation** pattern describes the case when the user knows exactly what they want,
+and they already have an address or id of the target file.
+BSFS identifies each file with a unique URI,
+or you can quickly navigate to a file via its name or other file properties.
+
+A **search** occurs when the user lacks the specific address or identifier to a target file,
+but they have relatively clear and narrow search criteria.
+With BSFS, you can search by file properties (name, size), content (keywords, features), 
+or associations to other files and auxiliary nodes (tags, collections).
+
+**Browsing** takes place when the user has only vague query criteria but wants to quickly scan and compare many files.
+In BSFS, you can browse along file associations and rank results by a variety of similarity metrics.
+
+.. toctree::
+   :maxdepth: 1
+
+   installation
+   concepts
+   architecture
+   api/modules
+
+
+.. [#f1] although links and similar techniques allow some deviation from this principle
+
+.. _find: https://www.gnu.org/software/findutils/manual/html_node/find_html/Invoking-find.html#Invoking-find
+
+.. _locate: https://www.gnu.org/software/findutils/manual/html_node/find_html/Invoking-locate.html
+
+
diff --git a/doc/source/installation.rst b/doc/source/installation.rst
new file mode 100644
index 0000000..4316136
--- /dev/null
+++ b/doc/source/installation.rst
@@ -0,0 +1,43 @@
+
+Installation
+============
+
+Install *BSFS* via pip::
+
+    pip install --extra-index-url https://pip.bsfs.io bsfs
+
+This installs the `bsfs` python package as well as the `bsfs.app` command.
+It is recommended to install *bsfs* in a virtual environment (via `virtualenv`).
+
+
+License
+-------
+
+This project is released under the terms of the 3-clause BSD License.
+By downloading or using the application you agree to the license's terms and conditions.
+
+.. literalinclude:: ../../LICENSE
+
+
+Source
+------
+
+Check out our git repository::
+
+    git clone https://git.bsfs.io/bsfs.git
+
+You can further install *bsfs* via the ususal `setuptools <https://setuptools.pypa.io/en/latest/index.html>`_ commands from your bsfs source directory::
+
+    python setup.py develop
+
+For development, you also need to install some additional dependencies::
+
+    # code style discipline
+    pip install mypy coverage pylint
+
+    # documentation
+    pip install sphinx sphinx-copybutton furo
+
+    # packaging
+    pip install build
+
diff --git a/setup.py b/setup.py
index ab3864a..f6bd3e8 100644
--- a/setup.py
+++ b/setup.py
@@ -1,23 +1,44 @@
 
-from setuptools import setup
+from setuptools import setup, find_packages
 import os
 
 setup(
+    # package metadata
     name='bsfs',
-    version='0.0.1',
+    version='0.23.03',
     author='Matthias Baumgartner',
-    author_email='dev@igsor.net',
-    description='A content aware graph file system.',
-    long_description=open(os.path.join(os.path.dirname(__file__), 'README')).read(),
+    author_email='dev@bsfs.io',
+    description='A content-aware graph file system.',
+    long_description=open(os.path.join(os.path.dirname(__file__), 'README.md')).read(),
     license='BSD',
     license_files=('LICENSE', ),
-    url='https://www.igsor.net/projects/blackstar/bsfs/',
-    download_url='https://pip.igsor.net',
-    packages=('bsfs', ),
-    install_requires=('rdflib', ),
-    python_requires=">=3.7",
-)
+    url='https://www.bsfs.io/bsfs/',
+    download_url='https://pip.bsfs.io',
+
+    # packages
+    packages=find_packages(include=['bsfs']),
+    package_dir={'bsfs': 'bsfs'},
+    # data files are included if mentioned in MANIFEST.in
+    include_package_data=True,
 
-# FIXME: bsfs/graph/schema.nt
-# FIXME: bsfs.app
+    # entrypoints
+    entry_points={
+        'console_scripts': [
+            'bsfs = bsfs.apps:main',
+            ],
+        },
 
+    # dependencies
+    python_requires=">=3.7",
+    install_requires=(
+        'rdflib', # schema and sparql storage
+        'hopcroftkarp', # ast matching
+        'numpy', # distance functions for sparql store
+        ),
+    extras_require={
+        'dev': ['coverage', 'mypy', 'pylint'],
+        'doc': ['sphinx', 'furo', 'sphinx-copybutton'],
+        'test': [],
+        'build': ['build'],
+        },
+)
diff --git a/test/apps/schema-1.nt b/test/apps/schema-1.nt
index e57146d..4daf0ad 100644
--- a/test/apps/schema-1.nt
+++ b/test/apps/schema-1.nt
@@ -3,8 +3,8 @@ prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
 prefix xsd: <http://www.w3.org/2001/XMLSchema#>
 
 # common bsfs prefixes
-prefix bsfs: <http://bsfs.ai/schema/>
-prefix bse: <http://bsfs.ai/schema/Entity#>
+prefix bsfs: <http://schema.bsfs.io/core/>
+prefix bse: <http://schema.bsfs.io/core/Node/Entity#>
 
 # essential nodes
 bsfs:Entity rdfs:subClassOf bsfs:Node .
diff --git a/test/apps/schema-2.nt b/test/apps/schema-2.nt
index 525ac99..4eb2467 100644
--- a/test/apps/schema-2.nt
+++ b/test/apps/schema-2.nt
@@ -3,14 +3,15 @@ prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
 prefix xsd: <http://www.w3.org/2001/XMLSchema#>
 
 # common bsfs prefixes
-prefix bsfs: <http://bsfs.ai/schema/>
-prefix bse: <http://bsfs.ai/schema/Entity#>
+prefix bsfs: <http://schema.bsfs.io/core/>
+prefix bse: <http://schema.bsfs.io/core/Node/Entity#>
 
 # essential nodes
 bsfs:Entity rdfs:subClassOf bsfs:Node .
 
 # common definitions
-xsd:integer rdfs:subClassOf bsfs:Literal .
+bsfs:Number rdfs:subClassOf bsfs:Literal .
+xsd:integer rdfs:subClassOf bsfs:Number .
 
 bse:filesize rdfs:subClassOf bsfs:Predicate ;
     rdfs:domain bsfs:Entity ;
diff --git a/test/apps/test_init.py b/test/apps/test_init.py
index bae6a68..59e10eb 100644
--- a/test/apps/test_init.py
+++ b/test/apps/test_init.py
@@ -1,9 +1,4 @@
-"""
 
-Part of the bsfs test suite.
-A copy of the license is provided with the project.
-Author: Matthias Baumgartner, 2022
-"""
 # imports
 import contextlib
 import io
diff --git a/test/apps/test_main.py b/test/apps/test_main.py
new file mode 100644
index 0000000..d61372f
--- /dev/null
+++ b/test/apps/test_main.py
@@ -0,0 +1,37 @@
+
+# imports
+import contextlib
+import io
+import json
+import unittest
+
+# objects to test
+from bsfs.apps import main
+
+
+## code ##
+
+class TestMain(unittest.TestCase):
+    def test_main(self):
+        # must at least pass an app
+        with contextlib.redirect_stderr(io.StringIO()):
+            self.assertRaises(SystemExit, main, [])
+        # app takes over
+        with contextlib.redirect_stderr(io.StringIO()):
+            self.assertRaises(SystemExit, main, ['init'])
+        outbuf = io.StringIO()
+        with contextlib.redirect_stdout(outbuf):
+            main(['init', 'sparql'])
+        self.assertEqual(json.loads(outbuf.getvalue()), {
+            'Graph': {
+                'user': 'http://example.com/me',
+                'backend': {
+                    'SparqlStore': {}}}})
+
+
+## main ##
+
+if __name__ == '__main__':
+    unittest.main()
+
+## EOF ##
diff --git a/test/apps/test_migrate.py b/test/apps/test_migrate.py
index 957509a..618cb37 100644
--- a/test/apps/test_migrate.py
+++ b/test/apps/test_migrate.py
@@ -1,9 +1,4 @@
-"""
 
-Part of the bsfs test suite.
-A copy of the license is provided with the project.
-Author: Matthias Baumgartner, 2022
-"""
 # imports
 import contextlib
 import io
@@ -13,7 +8,7 @@ import unittest
 import unittest.mock
 
 # bsie imports
-from bsfs.schema import Schema
+from bsfs import schema
 
 # objects to test
 from bsfs.apps.migrate import main
@@ -33,21 +28,21 @@ class TestMigrate(unittest.TestCase):
 
         # read schema from file
         with open(schema_1) as ifile:
-            target = Schema.from_string(ifile.read())
+            target = schema.from_string(ifile.read())
         graph = main([config, schema_1])
         self.assertTrue(target <= graph.schema)
 
         # read schema from multiple files
         with open(schema_1) as ifile:
-            target = Schema.from_string(ifile.read())
+            target = schema.from_string(ifile.read())
         with open(schema_2) as ifile:
-            target = target + Schema.from_string(ifile.read())
+            target = target + schema.from_string(ifile.read())
         graph = main([config, schema_1, schema_2])
         self.assertTrue(target <= graph.schema)
 
         # read schema from stdin
         with open(schema_1, 'rt') as ifile:
-            target = Schema.from_string(ifile.read())
+            target = schema.from_string(ifile.read())
         with open(schema_1, 'rt') as ifile:
             with unittest.mock.patch('sys.stdin', ifile):
                 graph = main([config])
diff --git a/test/front/test_bsfs.py b/test/front/test_bsfs.py
index 0d7f383..8905bf8 100644
--- a/test/front/test_bsfs.py
+++ b/test/front/test_bsfs.py
@@ -1,14 +1,10 @@
-"""
 
-Part of the bsfs test suite.
-A copy of the license is provided with the project.
-Author: Matthias Baumgartner, 2022
-"""
 # imports
 import unittest
 
 # bsie imports
 from bsfs.graph import Graph
+from bsfs.graph.ac import NullAC
 from bsfs.triple_store import SparqlStore
 from bsfs.utils import errors, URI
 
@@ -25,7 +21,7 @@ class TestBSFS(unittest.TestCase):
         graph = Open(config)
         self.assertIsInstance(graph, Graph)
         self.assertIsInstance(graph._backend, SparqlStore)
-        self.assertEqual(graph._user, URI('http://example.com/me'))
+        self.assertEqual(graph._ac, NullAC(graph._backend, URI('http://example.com/me')))
         # invalid config raises an error
         self.assertRaises(errors.ConfigError, Open, {})
 
diff --git a/test/front/test_builder.py b/test/front/test_builder.py
index 08f2027..875fa8a 100644
--- a/test/front/test_builder.py
+++ b/test/front/test_builder.py
@@ -1,14 +1,10 @@
-"""
 
-Part of the bsfs test suite.
-A copy of the license is provided with the project.
-Author: Matthias Baumgartner, 2022
-"""
 # imports
 import unittest
 
 # bsie imports
 from bsfs.graph import Graph
+from bsfs.graph.ac import NullAC
 from bsfs.triple_store import SparqlStore
 from bsfs.utils import errors, URI
 
@@ -40,7 +36,7 @@ class TestBuilder(unittest.TestCase):
         graph = build_graph({'Graph': {'backend': {'SparqlStore': {}}, 'user': 'http://example.com/me'}})
         self.assertIsInstance(graph, Graph)
         self.assertIsInstance(graph._backend, SparqlStore)
-        self.assertEqual(graph._user, URI('http://example.com/me'))
+        self.assertEqual(graph._ac, NullAC(graph._backend, URI('http://example.com/me')))
         # cannot create an invalid graph
         self.assertRaises(errors.ConfigError, build_graph, {'MyGraph': {}})
         # must pass a dict
diff --git a/test/graph/ac/test_base.py b/test/graph/ac/test_base.py
new file mode 100644
index 0000000..addecd4
--- /dev/null
+++ b/test/graph/ac/test_base.py
@@ -0,0 +1,78 @@
+
+# imports
+import unittest
+
+# bsie imports
+from bsfs import schema as bsc
+from bsfs.namespace import ns
+from bsfs.query import ast
+from bsfs.triple_store import SparqlStore
+from bsfs.utils import URI
+
+# objects to test
+from bsfs.graph.ac.base import AccessControlBase
+
+
+## code ##
+
+class StubAC(AccessControlBase):
+    def is_protected_predicate(self, pred):
+        pass
+    def create(self, node_type, guids):
+        pass
+    def link_from_node(self, node_type, guids):
+        pass
+    def link_to_node(self, node_type, guids):
+        pass
+    def write_literal(self, node_type, guids):
+        pass
+    def createable(self, node_type, guids):
+        pass
+    def filter_read(self, node_type, query):
+        pass
+    def fetch_read(self, node_type, query):
+        pass
+
+
+class TestAccessControlBase(unittest.TestCase):
+    def setUp(self):
+        self.backend = SparqlStore()
+        self.user = URI('http://www.example.com/me')
+
+    def test_essentials(self):
+        ac = StubAC(self.backend, self.user)
+        # equal construction means equal instance
+        self.assertEqual(StubAC(self.backend, self.user), StubAC(self.backend, self.user))
+        self.assertEqual(hash(StubAC(self.backend, self.user)), hash(StubAC(self.backend, self.user)))
+        self.assertEqual(ac, StubAC(self.backend, self.user))
+        self.assertEqual(hash(ac), hash(StubAC(self.backend, self.user)))
+        # equivalence respects type
+        class Foo(): pass
+        self.assertNotEqual(ac, 1234)
+        self.assertNotEqual(hash(ac), hash(1234))
+        self.assertNotEqual(ac, 'hello')
+        self.assertNotEqual(hash(ac), hash('hello'))
+        self.assertNotEqual(ac, Foo())
+        self.assertNotEqual(hash(ac), hash(Foo()))
+        # equivalence respects backend
+        self.assertNotEqual(ac, StubAC(SparqlStore(), self.user))
+        self.assertNotEqual(hash(ac), hash(StubAC(SparqlStore(), self.user)))
+        # equivalence respects user
+        self.assertNotEqual(ac, StubAC(self.backend, URI('http://www.example.com/you')))
+        self.assertNotEqual(hash(ac), hash(StubAC(self.backend, URI('http://www.example.com/you'))))
+        # string conversion
+        self.assertEqual(str(ac), f'StubAC({self.user})')
+        self.assertEqual(repr(ac), f'StubAC({self.user})')
+        # string conversion respects user
+        self.assertEqual(str(StubAC(self.backend, URI('http://www.example.com/you'))),
+            f'StubAC(http://www.example.com/you)')
+        self.assertEqual(repr(StubAC(self.backend, URI('http://www.example.com/you'))),
+            f'StubAC(http://www.example.com/you)')
+
+
+## main ##
+
+if __name__ == '__main__':
+    unittest.main()
+
+## EOF ##
diff --git a/test/graph/ac/test_null.py b/test/graph/ac/test_null.py
index f39c9be..142bc23 100644
--- a/test/graph/ac/test_null.py
+++ b/test/graph/ac/test_null.py
@@ -1,15 +1,11 @@
-"""
 
-Part of the bsfs test suite.
-A copy of the license is provided with the project.
-Author: Matthias Baumgartner, 2022
-"""
 # imports
 import unittest
 
 # bsie imports
-from bsfs import schema as _schema
+from bsfs import schema as bsc
 from bsfs.namespace import ns
+from bsfs.query import ast
 from bsfs.triple_store import SparqlStore
 from bsfs.utils import URI
 
@@ -19,24 +15,28 @@ from bsfs.graph.ac.null import NullAC
 
 ## code ##
 
+ns.bse = ns.bsfs.Entity()
+
 class TestNullAC(unittest.TestCase):
     def setUp(self):
         self.backend = SparqlStore()
-        self.backend.schema = _schema.Schema.from_string('''
+        self.backend.schema = bsc.from_string('''
             prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
             prefix xsd: <http://www.w3.org/2001/XMLSchema#>
 
-            prefix bsfs: <http://bsfs.ai/schema/>
-            prefix bsm: <http://bsfs.ai/schema/Meta#>
-            prefix bse: <http://bsfs.ai/schema/Entity#>
+            prefix bsfs: <https://schema.bsfs.io/core/>
+            prefix bsl: <https://schema.bsfs.io/core/Literal/>
+            prefix bsn: <https://schema.bsfs.io/core/Node#>
+            prefix bse: <https://schema.bsfs.io/core/Entity#>
 
             bsfs:Entity rdfs:subClassOf bsfs:Node .
             bsfs:Tag rdfs:subClassOf bsfs:Node .
             xsd:string rdfs:subClassOf bsfs:Literal .
-            xsd:integer rdfs:subClassOf bsfs:Literal .
+            bsl:Number rdfs:subClassOf bsfs:Literal .
+            xsd:integer rdfs:subClassOf bsl:Number .
 
             # predicates mandated by Nodes
-            bsm:t_created rdfs:subClassOf bsfs:Predicate ;
+            bsn:t_created rdfs:subClassOf bsfs:Predicate ;
                 rdfs:domain bsfs:Node ;
                 rdfs:range xsd:integer ;
                 bsfs:unique "true"^^xsd:boolean .
@@ -62,10 +62,40 @@ class TestNullAC(unittest.TestCase):
         self.p_author = self.backend.schema.predicate(ns.bse.author)
         self.p_filesize = self.backend.schema.predicate(ns.bse.filesize)
         self.p_tag = self.backend.schema.predicate(ns.bse.tag)
-        self.p_created = self.backend.schema.predicate(ns.bsm.t_created)
+        self.p_created = self.backend.schema.predicate(ns.bsn.t_created)
         self.ent_type = self.backend.schema.node(ns.bsfs.Entity)
         self.ent_ids = {URI('http://www.example.com/me/entity#1234'), URI('http://www.example.com/me/entity#4321')}
 
+    def test_essentials(self):
+        ac = NullAC(self.backend, self.user)
+        # equal construction means equal instance
+        self.assertEqual(NullAC(self.backend, self.user), NullAC(self.backend, self.user))
+        self.assertEqual(hash(NullAC(self.backend, self.user)), hash(NullAC(self.backend, self.user)))
+        self.assertEqual(ac, NullAC(self.backend, self.user))
+        self.assertEqual(hash(ac), hash(NullAC(self.backend, self.user)))
+        # equivalence respects type
+        class Foo(): pass
+        self.assertNotEqual(ac, 1234)
+        self.assertNotEqual(hash(ac), hash(1234))
+        self.assertNotEqual(ac, 'hello')
+        self.assertNotEqual(hash(ac), hash('hello'))
+        self.assertNotEqual(ac, Foo())
+        self.assertNotEqual(hash(ac), hash(Foo()))
+        # equivalence respects backend
+        self.assertNotEqual(ac, NullAC(SparqlStore(), self.user))
+        self.assertNotEqual(hash(ac), hash(NullAC(SparqlStore(), self.user)))
+        # equivalence respects user
+        self.assertNotEqual(ac, NullAC(self.backend, URI('http://www.example.com/you')))
+        self.assertNotEqual(hash(ac), hash(NullAC(self.backend, URI('http://www.example.com/you'))))
+        # string conversion
+        self.assertEqual(str(ac), f'NullAC({self.user})')
+        self.assertEqual(repr(ac), f'NullAC({self.user})')
+        # string conversion respects user
+        self.assertEqual(str(NullAC(self.backend, URI('http://www.example.com/you'))),
+            f'NullAC(http://www.example.com/you)')
+        self.assertEqual(repr(NullAC(self.backend, URI('http://www.example.com/you'))),
+            f'NullAC(http://www.example.com/you)')
+
     def test_is_protected_predicate(self):
         ac = NullAC(self.backend, self.user)
         self.assertTrue(ac.is_protected_predicate(self.p_created))
@@ -93,6 +123,26 @@ class TestNullAC(unittest.TestCase):
         ac = NullAC(self.backend, self.user)
         self.assertSetEqual(self.ent_ids, ac.createable(self.ent_type, self.ent_ids))
 
+    def test_filter_read(self):
+        query = ast.filter.Or(
+            ast.filter.Any(ns.bse.tag, ast.filter.Is('http://example.com/tag#1234')),
+            ast.filter.Any(ns.bse.tag, ast.filter.Is('http://example.com/tag#4321')),
+            ast.filter.Any(ns.bse.author, ast.filter.Equals('Me, Myself, and I')))
+        ac = NullAC(self.backend, self.user)
+        # NullAC returns query
+        self.assertEqual(query, ac.filter_read(self.ent_type, query))
+        # query can be none
+        self.assertIsNone(ac.filter_read(self.ent_type, None))
+
+    def test_fetch_read(self):
+        query = ast.fetch.All(
+            ast.fetch.Fetch(ns.bse.tag, ast.fetch.Value(ns.bse.label, 'tag_label')),
+            ast.fetch.Node(ns.bse.tag, 'tag_node'),
+            ast.fetch.Value(ns.bse.iso, 'iso'))
+        ac = NullAC(self.backend, self.user)
+        # NullAC returns query
+        self.assertEqual(query, ac.fetch_read(self.ent_type, query))
+
 
 ## main ##
 
diff --git a/test/graph/test_graph.py b/test/graph/test_graph.py
index 33cf6aa..167168d 100644
--- a/test/graph/test_graph.py
+++ b/test/graph/test_graph.py
@@ -1,18 +1,17 @@
-"""
 
-Part of the bsfs test suite.
-A copy of the license is provided with the project.
-Author: Matthias Baumgartner, 2022
-"""
 # imports
+from functools import reduce
+import operator
 import unittest
 
 # bsie imports
 from bsfs import schema
+from bsfs.graph.ac import NullAC
+from bsfs.graph.nodes import Nodes
 from bsfs.namespace import ns
+from bsfs.query import ast
 from bsfs.triple_store import SparqlStore
 from bsfs.utils import URI, errors
-from bsfs.graph.nodes import Nodes
 
 # objects to test
 from bsfs.graph.graph import Graph
@@ -20,83 +19,95 @@ from bsfs.graph.graph import Graph
 
 ## code ##
 
+ns.bse = ns.bsfs.Entity()
+
 class TestGraph(unittest.TestCase):
     def setUp(self):
-        self.user = URI('http://example.com/me')
         self.backend = SparqlStore.Open()
-        self.backend.schema = schema.Schema.from_string('''
+        self.backend.schema = schema.from_string('''
             prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
-            prefix bsfs: <http://bsfs.ai/schema/>
+            prefix bsfs: <https://schema.bsfs.io/core/>
             bsfs:Entity rdfs:subClassOf bsfs:Node .
             ''')
+        self.user = URI('http://example.com/me')
+        self.ac = NullAC(self.backend, self.user)
 
     def test_str(self):
-        self.assertEqual(str(Graph(self.backend, self.user)),
-            'Graph(SparqlStore(uri=None), http://example.com/me)')
-        self.assertEqual(repr(Graph(self.backend, self.user)),
-            'Graph(backend=SparqlStore(uri=None), user=http://example.com/me)')
+        self.assertEqual(str(Graph(self.backend, self.ac)),
+            'Graph(SparqlStore(uri=None))')
+        self.assertEqual(repr(Graph(self.backend, self.ac)),
+            'Graph(SparqlStore(uri=None), NullAC(http://example.com/me))')
         # str respects backend
         class Foo(SparqlStore): pass
-        self.assertEqual(str(Graph(Foo.Open(), self.user)),
-            'Graph(Foo(uri=None), http://example.com/me)')
-        self.assertEqual(repr(Graph(Foo.Open(), self.user)),
-            'Graph(backend=Foo(uri=None), user=http://example.com/me)')
+        self.assertEqual(str(Graph(Foo.Open(), self.ac)),
+            'Graph(Foo(uri=None))')
+        self.assertEqual(repr(Graph(Foo.Open(), self.ac)),
+            'Graph(Foo(uri=None), NullAC(http://example.com/me))')
         # str respect user
-        self.assertEqual(str(Graph(self.backend, URI('http://example.com/you'))),
-            'Graph(SparqlStore(uri=None), http://example.com/you)')
-        self.assertEqual(repr(Graph(self.backend, URI('http://example.com/you'))),
-            'Graph(backend=SparqlStore(uri=None), user=http://example.com/you)')
+        self.assertEqual(str(Graph(self.backend, NullAC(self.backend, URI('http://example.com/you')))),
+            'Graph(SparqlStore(uri=None))')
+        self.assertEqual(repr(Graph(self.backend, NullAC(self.backend, URI('http://example.com/you')))),
+            'Graph(SparqlStore(uri=None), NullAC(http://example.com/you))')
         # str respects type
         class Bar(Graph): pass
-        self.assertEqual(str(Bar(self.backend, self.user)),
-            'Bar(SparqlStore(uri=None), http://example.com/me)')
-        self.assertEqual(repr(Bar(self.backend, self.user)),
-            'Bar(backend=SparqlStore(uri=None), user=http://example.com/me)')
+        self.assertEqual(str(Bar(self.backend, self.ac)),
+            'Bar(SparqlStore(uri=None))')
+        self.assertEqual(repr(Bar(self.backend, self.ac)),
+            'Bar(SparqlStore(uri=None), NullAC(http://example.com/me))')
 
     def test_equality(self):
-        graph = Graph(self.backend, self.user)
+        graph = Graph(self.backend, self.ac)
         # instance is equal to itself
         self.assertEqual(graph, graph)
         self.assertEqual(hash(graph), hash(graph))
         # instance is equal to a clone
-        self.assertEqual(graph, Graph(self.backend, self.user))
-        self.assertEqual(hash(graph), hash(Graph(self.backend, self.user)))
+        self.assertEqual(graph, Graph(self.backend, self.ac))
+        self.assertEqual(hash(graph), hash(Graph(self.backend, self.ac)))
         # equality respects backend
-        self.assertNotEqual(graph, Graph(SparqlStore.Open(), self.user))
-        self.assertNotEqual(hash(graph), hash(Graph(SparqlStore.Open(), self.user)))
+        self.assertNotEqual(graph, Graph(SparqlStore.Open(), self.ac))
+        self.assertNotEqual(hash(graph), hash(Graph(SparqlStore.Open(), self.ac)))
         # equality respects user
         self.assertNotEqual(graph, Graph(self.backend, URI('http://example.com/you')))
         self.assertNotEqual(hash(graph), hash(Graph(self.backend, URI('http://example.com/you'))))
 
     def test_essentials(self):
-        graph = Graph(self.backend, self.user)
+        graph = Graph(self.backend, self.ac)
         # schema
         self.assertEqual(graph.schema, self.backend.schema)
         self.assertRaises(AttributeError, setattr, graph, 'schema', None)
 
     def test_node(self):
-        graph = Graph(self.backend, self.user)
+        graph = Graph(self.backend, self.ac)
         guid = URI('http://example.com/me/entity#1234')
         # returns a Nodes instance
         self.assertEqual(
             graph.node(ns.bsfs.Entity, guid),
-            Nodes(self.backend, self.user, graph.schema.node(ns.bsfs.Entity), {guid}))
+            Nodes(self.backend, self.ac, graph.schema.node(ns.bsfs.Entity), {guid}))
         # node_type must be in the schema
         self.assertRaises(KeyError, graph.node, ns.bsfs.Invalid, guid)
 
     def test_nodes(self):
-        graph = Graph(self.backend, self.user)
+        graph = Graph(self.backend, self.ac)
         guids = {URI('http://example.com/me/entity#1234'), URI('http://example.com/me/entity#4321')}
         # returns a Nodes instance
         self.assertEqual(
             graph.nodes(ns.bsfs.Entity, guids),
-            Nodes(self.backend, self.user, graph.schema.node(ns.bsfs.Entity), guids))
+            Nodes(self.backend, self.ac, graph.schema.node(ns.bsfs.Entity), guids))
         # node_type must be in the schema
         self.assertRaises(KeyError, graph.nodes, ns.bsfs.Invalid, guids)
 
+    def test_empty(self):
+        graph = Graph(self.backend, self.ac)
+        # returns a Nodes instance
+        self.assertEqual(
+            graph.empty(ns.bsfs.Entity),
+            Nodes(self.backend, self.ac, graph.schema.node(ns.bsfs.Entity), set()))
+        # node_type must be in the schema
+        self.assertRaises(KeyError, graph.empty, ns.bsfs.Invalid)
+
     def test_migrate(self):
         # setup
-        graph = Graph(self.backend, self.user)
+        graph = Graph(self.backend, self.ac)
 
         # argument must be a schema
         class Foo(): pass
@@ -117,14 +128,16 @@ class TestGraph(unittest.TestCase):
                     schema.Node(ns.bsfs.Node, None)))}), append=False)
 
         # can migrate to compatible schema
-        target_1 = schema.Schema.from_string('''
+        target_1 = schema.from_string('''
             prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
             prefix xsd: <http://www.w3.org/2001/XMLSchema#>
-            prefix bsfs: <http://bsfs.ai/schema/>
-            prefix bse: <http://bsfs.ai/schema/Entity#>
+            prefix bsfs: <https://schema.bsfs.io/core/>
+            prefix bse: <https://schema.bsfs.io/core/Entity#>
+            prefix bsl: <https://schema.bsfs.io/core/Literal/>
             bsfs:Entity rdfs:subClassOf bsfs:Node .
             xsd:string rdfs:subClassOf bsfs:Literal .
-            xsd:integer rdfs:subClassOf bsfs:Literal .
+            bsl:Number rdfs:subClassOf bsfs:Literal .
+            xsd:integer rdfs:subClassOf bsl:Number .
 
             bse:filename rdfs:subClassOf bsfs:Predicate ;
                 rdfs:domain bsfs:Entity ;
@@ -141,27 +154,31 @@ class TestGraph(unittest.TestCase):
         # new schema is applied
         self.assertLess(target_1, graph.schema)
         # graph appends its predicates
-        self.assertEqual(graph.schema, target_1 + schema.Schema.from_string('''
+        self.assertEqual(graph.schema, target_1 + schema.from_string('''
             prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
             prefix xsd: <http://www.w3.org/2001/XMLSchema#>
-            prefix bsfs: <http://bsfs.ai/schema/>
-            prefix bsm: <http://bsfs.ai/schema/Meta#>
-            xsd:integer rdfs:subClassOf bsfs:Literal .
-            bsm:t_created rdfs:subClassOf bsfs:Predicate ;
+            prefix bsfs: <https://schema.bsfs.io/core/>
+            prefix bsn: <https://schema.bsfs.io/core/Node#>
+            prefix bsl: <https://schema.bsfs.io/core/Literal/>
+            bsl:Number rdfs:subClassOf bsfs:Literal .
+            xsd:float rdfs:subClassOf bsl:Number .
+            bsn:t_created rdfs:subClassOf bsfs:Predicate ;
                 rdfs:domain bsfs:Node ;
-                rdfs:range xsd:integer ;
+                rdfs:range xsd:float ;
                 bsfs:unique "true"^^xsd:boolean .
             '''))
 
         # can overwrite the current schema
-        target_2 = schema.Schema.from_string('''
+        target_2 = schema.from_string('''
             prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
             prefix xsd: <http://www.w3.org/2001/XMLSchema#>
-            prefix bsfs: <http://bsfs.ai/schema/>
-            prefix bse: <http://bsfs.ai/schema/Entity#>
+            prefix bsfs: <http://schema.bsfs.io/core/>
+            prefix bse: <http://schema.bsfs.io/core/Node/Entity#>
+            prefix bsl: <https://schema.bsfs.io/core/Literal/>
             bsfs:Entity rdfs:subClassOf bsfs:Node .
             xsd:string rdfs:subClassOf bsfs:Literal .
-            xsd:integer rdfs:subClassOf bsfs:Literal .
+            bsl:Number rdfs:subClassOf bsfs:Literal .
+            xsd:integer rdfs:subClassOf bsl:Number .
 
             bse:filename rdfs:subClassOf bsfs:Predicate ;
                 rdfs:domain bsfs:Entity ;
@@ -180,18 +197,147 @@ class TestGraph(unittest.TestCase):
         # new schema is applied
         self.assertLess(target_2, graph.schema)
         # graph appends its predicates
-        self.assertEqual(graph.schema, target_2 + schema.Schema.from_string('''
+        self.assertEqual(graph.schema, target_2 + schema.from_string('''
             prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
             prefix xsd: <http://www.w3.org/2001/XMLSchema#>
-            prefix bsfs: <http://bsfs.ai/schema/>
-            prefix bsm: <http://bsfs.ai/schema/Meta#>
-            xsd:integer rdfs:subClassOf bsfs:Literal .
-            bsm:t_created rdfs:subClassOf bsfs:Predicate ;
+            prefix bsfs: <https://schema.bsfs.io/core/>
+            prefix bsn: <https://schema.bsfs.io/core/Node#>
+            prefix bsl: <https://schema.bsfs.io/core/Literal/>
+            bsl:Number rdfs:subClassOf bsfs:Literal .
+            xsd:float rdfs:subClassOf bsl:Number .
+            bsn:t_created rdfs:subClassOf bsfs:Predicate ;
                 rdfs:domain bsfs:Node ;
-                rdfs:range xsd:integer ;
+                rdfs:range xsd:float ;
                 bsfs:unique "true"^^xsd:boolean .
             '''))
 
+    def test_get(self):
+        # setup
+        graph = Graph(self.backend, self.ac)
+        graph.migrate(schema.from_string('''
+            prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
+            prefix xsd: <http://www.w3.org/2001/XMLSchema#>
+            prefix bsfs: <https://schema.bsfs.io/core/>
+            prefix bse: <https://schema.bsfs.io/core/Entity#>
+
+            bsfs:Entity rdfs:subClassOf bsfs:Node .
+            bsfs:Tag rdfs:subClassOf bsfs:Node .
+            xsd:string rdfs:subClassOf bsfs:Literal .
+
+            bse:tag rdfs:subClassOf bsfs:Predicate ;
+                rdfs:domain bsfs:Entity ;
+                rdfs:range bsfs:Tag ;
+                bsfs:unique "false"^^xsd:boolean .
+
+            bse:comment rdfs:subClassOf bsfs:Predicate ;
+                rdfs:domain bsfs:Node ;
+                rdfs:range xsd:string ;
+                bsfs:unique "false"^^xsd:boolean .
+
+            '''))
+        # add some instances
+        ents = graph.nodes(ns.bsfs.Entity, {URI('http://example.com/entity#1234'), URI('http://example.com/entity#4321')})
+        tags = graph.nodes(ns.bsfs.Tag, {URI('http://example.com/tag#1234'), URI('http://example.com/tag#4321')})
+        # add some node links
+        ents.set(ns.bse.tag, tags)
+        # add some literals
+        graph.node(ns.bsfs.Entity, URI('http://example.com/entity#1234')).set(ns.bse.comment, 'hello world')
+        graph.node(ns.bsfs.Entity, URI('http://example.com/entity#1234')).set(ns.bse.comment, 'foo')
+        graph.node(ns.bsfs.Entity, URI('http://example.com/entity#1234')).set(ns.bse.comment, 'foobar')
+        graph.node(ns.bsfs.Tag, URI('http://example.com/tag#1234')).set(ns.bse.comment, 'foo')
+        graph.node(ns.bsfs.Tag, URI('http://example.com/tag#4321')).set(ns.bse.comment, 'bar')
+
+        # invalid query raises exception
+        self.assertRaises(errors.ConsistencyError, graph.get, ns.bsfs.Entity, ast.filter.Any(ns.bse.tag, ast.filter.Equals('hello world')))
+
+        # get returns nodes
+        self.assertEqual(graph.get(ns.bsfs.Entity, ast.filter.Any(ns.bse.tag, ast.filter.Is(tags))), ents)
+        self.assertEqual(graph.get(ns.bsfs.Entity, ast.filter.Any(ns.bse.comment, ast.filter.StartsWith('foo'))),
+            graph.node(ns.bsfs.Entity, URI('http://example.com/entity#1234')))
+        self.assertEqual(graph.get(ns.bsfs.Node, ast.filter.Any(ns.bse.comment, ast.filter.StartsWith('foo'))),
+            graph.nodes(ns.bsfs.Node, {URI('http://example.com/entity#1234'), URI('http://example.com/tag#1234')}))
+        self.assertEqual(graph.get(ns.bsfs.Entity, ast.filter.Or(
+            ast.filter.Any(ns.bse.comment, ast.filter.EndsWith('bar')),
+            ast.filter.Any(ns.bse.tag, ast.filter.All(ns.bse.comment, ast.filter.Equals('bar'))))),
+            ents)
+
+        # query can be None
+        self.assertEqual(graph.get(ns.bsfs.Entity, None), ents)
+
+    def test_sorted(self):
+        # setup
+        graph = Graph(self.backend, self.ac)
+        graph.migrate(schema.from_string('''
+            prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
+            prefix xsd: <http://www.w3.org/2001/XMLSchema#>
+            prefix bsfs: <https://schema.bsfs.io/core/>
+            prefix bse: <https://schema.bsfs.io/core/Entity#>
+
+            bsfs:Entity rdfs:subClassOf bsfs:Node .
+            bsfs:Tag rdfs:subClassOf bsfs:Node .
+            xsd:string rdfs:subClassOf bsfs:Literal .
+
+            bse:tag rdfs:subClassOf bsfs:Predicate ;
+                rdfs:domain bsfs:Entity ;
+                rdfs:range bsfs:Tag ;
+                bsfs:unique "false"^^xsd:boolean .
+
+            bse:comment rdfs:subClassOf bsfs:Predicate ;
+                rdfs:domain bsfs:Node ;
+                rdfs:range xsd:string ;
+                bsfs:unique "false"^^xsd:boolean .
+
+            '''))
+        # add some instances
+        ents = [
+            # default is alphabetical order
+            graph.node(ns.bsfs.Entity, URI('http://example.com/entity#1234')),
+            graph.node(ns.bsfs.Entity, URI('http://example.com/entity#4321')),
+            ]
+        tags = graph.nodes(ns.bsfs.Tag, {URI('http://example.com/tag#1234'), URI('http://example.com/tag#4321')})
+        # add some node links
+        reduce(operator.add, ents).set(ns.bse.tag, tags)
+        # add some literals
+        graph.node(ns.bsfs.Entity, URI('http://example.com/entity#1234')).set(ns.bse.comment, 'hello world')
+        graph.node(ns.bsfs.Entity, URI('http://example.com/entity#1234')).set(ns.bse.comment, 'foo')
+        graph.node(ns.bsfs.Entity, URI('http://example.com/entity#1234')).set(ns.bse.comment, 'foobar')
+        graph.node(ns.bsfs.Tag, URI('http://example.com/tag#1234')).set(ns.bse.comment, 'foo')
+        graph.node(ns.bsfs.Tag, URI('http://example.com/tag#4321')).set(ns.bse.comment, 'bar')
+
+        # invalid query raises exception
+        self.assertRaises(errors.ConsistencyError, list, graph.sorted(ns.bsfs.Entity, ast.filter.Any(ns.bse.tag, ast.filter.Equals('hello world'))))
+
+        # get returns nodes
+        self.assertListEqual(list(graph.sorted(ns.bsfs.Entity, ast.filter.Any(ns.bse.tag, ast.filter.Is(tags)))), ents)
+        self.assertListEqual(list(graph.sorted(ns.bsfs.Entity, ast.filter.Any(ns.bse.comment, ast.filter.StartsWith('foo')))),
+            [graph.node(ns.bsfs.Entity, URI('http://example.com/entity#1234'))])
+        self.assertListEqual(list(graph.sorted(ns.bsfs.Node, ast.filter.Any(ns.bse.comment, ast.filter.StartsWith('foo')))), [
+            graph.node(ns.bsfs.Node, URI('http://example.com/entity#1234')),
+            graph.node(ns.bsfs.Node, URI('http://example.com/tag#1234')),
+            ])
+        self.assertListEqual(list(graph.sorted(ns.bsfs.Entity, ast.filter.Or(
+            ast.filter.Any(ns.bse.comment, ast.filter.EndsWith('bar')),
+            ast.filter.Any(ns.bse.tag, ast.filter.All(ns.bse.comment, ast.filter.Equals('bar')))))),
+            ents)
+
+        # query can be None
+        self.assertListEqual(list(graph.sorted(ns.bsfs.Entity, None)), ents)
+
+
+    def test_all(self):
+        graph = Graph(self.backend, self.ac)
+        # resulting nodes can be empty
+        self.assertEqual(graph.all(ns.bsfs.Entity),
+            Nodes(self.backend, self.ac, graph.schema.node(ns.bsfs.Entity), set()))
+        # resulting nodes contains all nodes of the respective type
+        guids = {URI('http://example.com/me/entity#1234'), URI('http://example.com/me/entity#4321')}
+        self.backend.create(graph.schema.node(ns.bsfs.Entity), guids)
+        self.assertEqual(graph.all(ns.bsfs.Entity),
+            Nodes(self.backend, self.ac, graph.schema.node(ns.bsfs.Entity), guids))
+        # node_type must be in the schema
+        self.assertRaises(KeyError, graph.all, ns.bsfs.Invalid)
+
+
 
 ## main ##
 
diff --git a/test/graph/test_nodes.py b/test/graph/test_nodes.py
index 43e7f6f..afe7522 100644
--- a/test/graph/test_nodes.py
+++ b/test/graph/test_nodes.py
@@ -1,16 +1,17 @@
-"""
 
-Part of the bsfs test suite.
-A copy of the license is provided with the project.
-Author: Matthias Baumgartner, 2022
-"""
-# imports
-import rdflib
+# standard imports
+from functools import partial
+import operator
 import unittest
 
+# external imports
+import rdflib
+
 # bsie imports
-from bsfs import schema as _schema
-from bsfs.namespace import ns
+from bsfs import schema as bsc
+from bsfs.graph.ac import NullAC
+from bsfs.graph.walk import Walk
+from bsfs.namespace import Namespace, ns
 from bsfs.triple_store.sparql import SparqlStore
 from bsfs.utils import errors, URI
 
@@ -20,27 +21,32 @@ from bsfs.graph.nodes import Nodes
 
 ## code ##
 
+ns.bse = ns.bsfs.Entity()
+ns.bst = ns.bsfs.Tag()
+
 class TestNodes(unittest.TestCase):
     def setUp(self):
         # initialize backend
         self.backend = SparqlStore()
-        self.backend.schema = _schema.Schema.from_string('''
+        self.backend.schema = bsc.from_string('''
             prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
             prefix xsd: <http://www.w3.org/2001/XMLSchema#>
 
-            prefix bsfs: <http://bsfs.ai/schema/>
-            prefix bsm: <http://bsfs.ai/schema/Meta#>
-            prefix bse: <http://bsfs.ai/schema/Entity#>
-            prefix bst: <http://bsfs.ai/schema/Tag#>
+            prefix bsfs: <https://schema.bsfs.io/core/>
+            prefix bsl: <https://schema.bsfs.io/core/Literal/>
+            prefix bsn: <https://schema.bsfs.io/core/Node#>
+            prefix bse: <https://schema.bsfs.io/core/Entity#>
+            prefix bst: <https://schema.bsfs.io/core/Tag#>
 
             bsfs:Entity rdfs:subClassOf bsfs:Node .
             bsfs:Tag rdfs:subClassOf bsfs:Node .
             bsfs:User rdfs:subClassOf bsfs:Node .
             xsd:string rdfs:subClassOf bsfs:Literal .
-            xsd:integer rdfs:subClassOf bsfs:Literal .
+            bsl:Number rdfs:subClassOf bsfs:Literal .
+            xsd:integer rdfs:subClassOf bsl:Number .
 
             # predicates mandated by Nodes
-            bsm:t_created rdfs:subClassOf bsfs:Predicate ;
+            bsn:t_created rdfs:subClassOf bsfs:Predicate ;
                 rdfs:domain bsfs:Node ;
                 rdfs:range xsd:integer ;
                 bsfs:unique "true"^^xsd:boolean .
@@ -66,14 +72,40 @@ class TestNodes(unittest.TestCase):
                 rdfs:range bsfs:User ;
                 bsfs:unique "true"^^xsd:boolean .
 
+            bst:label rdfs:subClassOf bsfs:Predicate ;
+                rdfs:domain bsfs:Tag ;
+                rdfs:range xsd:string ;
+                bsfs:unique "true"^^xsd:boolean .
+
             bst:representative rdfs:subClassOf bsfs:Predicate ;
                 rdfs:domain bsfs:Tag ;
                 rdfs:range bsfs:Entity ;
                 bsfs:unique "true"^^xsd:boolean .
 
             ''')
+        self.schema_triples = {
+            # schema hierarchy
+            (rdflib.URIRef(ns.bsfs.Entity), rdflib.RDFS.subClassOf, rdflib.URIRef(ns.bsfs.Node)),
+            (rdflib.URIRef(ns.bsfs.Tag), rdflib.RDFS.subClassOf, rdflib.URIRef(ns.bsfs.Node)),
+            (rdflib.URIRef(ns.bsfs.User), rdflib.RDFS.subClassOf, rdflib.URIRef(ns.bsfs.Node)),
+            (rdflib.URIRef(ns.xsd.string), rdflib.RDFS.subClassOf, rdflib.URIRef(ns.bsfs.Literal)),
+            (rdflib.URIRef(ns.bsl.Array), rdflib.RDFS.subClassOf, rdflib.URIRef(ns.bsfs.Literal)),
+            (rdflib.URIRef(ns.bsl.BinaryBlob), rdflib.RDFS.subClassOf, rdflib.URIRef(ns.bsfs.Literal)),
+            (rdflib.URIRef(ns.bsl.Array.Feature), rdflib.RDFS.subClassOf, rdflib.URIRef(ns.bsl.Array)),
+            (rdflib.URIRef(ns.bsl.Number), rdflib.RDFS.subClassOf, rdflib.URIRef(ns.bsfs.Literal)),
+            (rdflib.URIRef(ns.bsl.Time), rdflib.RDFS.subClassOf, rdflib.URIRef(ns.bsfs.Literal)),
+            (rdflib.URIRef(ns.xsd.integer), rdflib.RDFS.subClassOf, rdflib.URIRef(ns.bsl.Number)),
+            (rdflib.URIRef(ns.bsn.t_created), rdflib.RDFS.subClassOf, rdflib.URIRef(ns.bsfs.Predicate)),
+            (rdflib.URIRef(ns.bse.comment), rdflib.RDFS.subClassOf, rdflib.URIRef(ns.bsfs.Predicate)),
+            (rdflib.URIRef(ns.bse.filesize), rdflib.RDFS.subClassOf, rdflib.URIRef(ns.bsfs.Predicate)),
+            (rdflib.URIRef(ns.bse.tag), rdflib.RDFS.subClassOf, rdflib.URIRef(ns.bsfs.Predicate)),
+            (rdflib.URIRef(ns.bse.author), rdflib.RDFS.subClassOf, rdflib.URIRef(ns.bsfs.Predicate)),
+            (rdflib.URIRef(ns.bst.representative), rdflib.RDFS.subClassOf, rdflib.URIRef(ns.bsfs.Predicate)),
+            (rdflib.URIRef(ns.bst.label), rdflib.RDFS.subClassOf, rdflib.URIRef(ns.bsfs.Predicate)),
+            }
         # Nodes constructor args
         self.user = URI('http://example.com/me')
+        self.ac = NullAC(self.backend, self.user)
         # set args
         self.tag_type = self.backend.schema.node(ns.bsfs.Tag)
         self.ent_type = self.backend.schema.node(ns.bsfs.Entity)
@@ -81,8 +113,9 @@ class TestNodes(unittest.TestCase):
         self.p_filesize = self.backend.schema.predicate(ns.bse.filesize)
         self.p_author = self.backend.schema.predicate(ns.bse.author)
         self.p_tag = self.backend.schema.predicate(ns.bse.tag)
-        self.p_representative = self.backend.schema.predicate(URI('http://bsfs.ai/schema/Tag#representative'))
-        self.t_created = self.backend.schema.predicate(ns.bsm.t_created)
+        self.p_representative = self.backend.schema.predicate(ns.bst.representative)
+        self.p_label = self.backend.schema.predicate(ns.bst.label)
+        self.t_created = self.backend.schema.predicate(ns.bsn.t_created)
         self.ent_ids = {
             URI('http://example.com/me/entity#1234'),
             URI('http://example.com/me/entity#4321'),
@@ -92,67 +125,71 @@ class TestNodes(unittest.TestCase):
             URI('http://example.com/me/tag#4321'),
             }
 
+    def test_construct(self):
+        self.assertIsInstance(Nodes(self.backend, self.ac, self.ent_type, {'http://example.com/me-and-you'}), Nodes)
+        self.assertRaises(ValueError, Nodes, self.backend, self.ac, self.ent_type, {'http://example.com/me and you'})
+
     def test_str(self):
         # str baseline
-        nodes = Nodes(self.backend, self.user, self.ent_type, self.ent_ids)
-        self.assertEqual(str(nodes), f'Nodes({self.ent_type}, {self.ent_ids})')
-        self.assertEqual(repr(nodes), f'Nodes({self.backend}, {self.user}, {self.ent_type}, {self.ent_ids})')
+        nodes = Nodes(self.backend, self.ac, self.ent_type, {URI('http://example.com/me/entity#1234')})
+        self.assertEqual(str(nodes), f"Nodes({self.ent_type}, {{'http://example.com/me/entity#1234'}})")
+        self.assertEqual(repr(nodes), f"Nodes({self.backend}, {self.ac}, {self.ent_type}, {{'http://example.com/me/entity#1234'}})")
         # str respects node_type
-        nodes = Nodes(self.backend, self.user, self.tag_type, self.tag_ids)
-        self.assertEqual(str(nodes), f'Nodes({self.tag_type}, {self.tag_ids})')
-        self.assertEqual(repr(nodes), f'Nodes({self.backend}, {self.user}, {self.tag_type}, {self.tag_ids})')
+        nodes = Nodes(self.backend, self.ac, self.tag_type, {URI('http://example.com/me/tag#1234')})
+        self.assertEqual(str(nodes), f"Nodes({self.tag_type}, {{'http://example.com/me/tag#1234'}})")
+        self.assertEqual(repr(nodes), f"Nodes({self.backend}, {self.ac}, {self.tag_type}, {{'http://example.com/me/tag#1234'}})")
         # str respects guids
-        nodes = Nodes(self.backend, self.user, self.ent_type, {URI('http://example.com/me/entity#foo')})
+        nodes = Nodes(self.backend, self.ac, self.ent_type, {URI('http://example.com/me/entity#foo')})
         self.assertEqual(str(nodes), f'Nodes({self.ent_type}, {{\'http://example.com/me/entity#foo\'}})')
-        self.assertEqual(repr(nodes), f'Nodes({self.backend}, {self.user}, {self.ent_type}, {{\'http://example.com/me/entity#foo\'}})')
+        self.assertEqual(repr(nodes), f'Nodes({self.backend}, {self.ac}, {self.ent_type}, {{\'http://example.com/me/entity#foo\'}})')
         # repr respects backend
         class Foo(SparqlStore): pass
         backend = Foo.Open()
         backend.schema = self.backend.schema
-        nodes = Nodes(backend, self.user, self.ent_type, self.ent_ids)
-        self.assertEqual(repr(nodes), f'Nodes({backend}, {self.user}, {self.ent_type}, {self.ent_ids})')
+        nodes = Nodes(backend, self.ac, self.ent_type, {URI('http://example.com/me/entity#1234')})
+        self.assertEqual(repr(nodes), f"Nodes({backend}, {self.ac}, {self.ent_type}, {{'http://example.com/me/entity#1234'}})")
         # repr respects user
-        nodes = Nodes(self.backend, URI('http://example.com/you'), self.ent_type, self.ent_ids)
-        self.assertEqual(repr(nodes), f'Nodes({self.backend}, http://example.com/you, {self.ent_type}, {self.ent_ids})')
+        nodes = Nodes(self.backend, NullAC(self.backend, URI('http://example.com/you')), self.ent_type, {URI('http://example.com/me/entity#1234')})
+        self.assertEqual(repr(nodes), f"Nodes({self.backend}, NullAC(http://example.com/you), {self.ent_type}, {{'http://example.com/me/entity#1234'}})")
 
     def test_equality(self):
-        nodes = Nodes(self.backend, self.user, self.ent_type, self.ent_ids)
+        nodes = Nodes(self.backend, self.ac, self.ent_type, self.ent_ids)
         # instance is equal to itself
         self.assertEqual(nodes, nodes)
         self.assertEqual(hash(nodes), hash(nodes))
         # instance is equal to a clone
-        self.assertEqual(nodes, Nodes(self.backend, self.user, self.ent_type, self.ent_ids))
-        self.assertEqual(Nodes(self.backend, self.user, self.ent_type, self.ent_ids), nodes)
-        self.assertEqual(hash(nodes), hash(Nodes(self.backend, self.user, self.ent_type, self.ent_ids)))
+        self.assertEqual(nodes, Nodes(self.backend, self.ac, self.ent_type, self.ent_ids))
+        self.assertEqual(Nodes(self.backend, self.ac, self.ent_type, self.ent_ids), nodes)
+        self.assertEqual(hash(nodes), hash(Nodes(self.backend, self.ac, self.ent_type, self.ent_ids)))
         # equality respects backend
         backend = SparqlStore.Open()
         backend.schema = self.backend.schema
-        self.assertNotEqual(nodes, Nodes(backend, self.user, self.ent_type, self.ent_ids))
-        self.assertNotEqual(hash(nodes), hash(Nodes(backend, self.user, self.ent_type, self.ent_ids)))
+        self.assertNotEqual(nodes, Nodes(backend, self.ac, self.ent_type, self.ent_ids))
+        self.assertNotEqual(hash(nodes), hash(Nodes(backend, self.ac, self.ent_type, self.ent_ids)))
         # equality respects user
-        self.assertNotEqual(nodes, Nodes(self.backend, URI('http://example.com/you'), self.ent_type, self.ent_ids))
-        self.assertNotEqual(hash(nodes), hash(Nodes(self.backend, URI('http://example.com/you'), self.ent_type, self.ent_ids)))
+        self.assertNotEqual(nodes, Nodes(self.backend, NullAC(self.backend, URI('http://example.com/you')), self.ent_type, self.ent_ids))
+        self.assertNotEqual(hash(nodes), hash(Nodes(self.backend, NullAC(self.backend, URI('http://example.com/you')), self.ent_type, self.ent_ids)))
         # equality respects node_type
-        self.assertNotEqual(nodes, Nodes(self.backend, self.user, self.tag_type, self.ent_ids))
-        self.assertNotEqual(hash(nodes), hash(Nodes(self.backend, self.user, self.tag_type, self.ent_ids)))
+        self.assertNotEqual(nodes, Nodes(self.backend, self.ac, self.tag_type, self.ent_ids))
+        self.assertNotEqual(hash(nodes), hash(Nodes(self.backend, self.ac, self.tag_type, self.ent_ids)))
         # equality respects guids
-        self.assertNotEqual(nodes, Nodes(self.backend, self.user, self.ent_type, self.tag_ids))
-        self.assertNotEqual(hash(nodes), hash(Nodes(self.backend, self.user, self.ent_type, self.tag_ids)))
+        self.assertNotEqual(nodes, Nodes(self.backend, self.ac, self.ent_type, self.tag_ids))
+        self.assertNotEqual(hash(nodes), hash(Nodes(self.backend, self.ac, self.ent_type, self.tag_ids)))
 
     def test_properties(self):
         # node_type
         self.assertEqual(self.ent_type, Nodes(
-            self.backend, self.user, self.ent_type, self.ent_ids).node_type)
+            self.backend, self.ac, self.ent_type, self.ent_ids).node_type)
         self.assertEqual(self.tag_type, Nodes(
-            self.backend, self.user, self.tag_type, self.tag_ids).node_type)
+            self.backend, self.ac, self.tag_type, self.tag_ids).node_type)
         # guids
         self.assertSetEqual(self.ent_ids, set(Nodes(
-            self.backend, self.user, self.ent_type, self.ent_ids).guids))
+            self.backend, self.ac, self.ent_type, self.ent_ids).guids))
         self.assertSetEqual(self.tag_ids, set(Nodes(
-            self.backend, self.user, self.tag_type, self.tag_ids).guids))
+            self.backend, self.ac, self.tag_type, self.tag_ids).guids))
 
     def test__ensure_nodes(self):
-        nodes = Nodes(self.backend, self.user, self.ent_type, self.ent_ids)
+        nodes = Nodes(self.backend, self.ac, self.ent_type, self.ent_ids)
 
         # missing nodes are created
         self.assertSetEqual(self.ent_ids, nodes._ensure_nodes(self.ent_type, self.ent_ids))
@@ -160,10 +197,10 @@ class TestNodes(unittest.TestCase):
         time_triples = list(self.backend._graph.objects(rdflib.URIRef('http://example.com/me/entity#1234'), rdflib.URIRef(self.t_created.uri)))
         t_ent_created = float(time_triples[0]) if len(time_triples) > 0 else 0.0
         # check triples
-        self.assertSetEqual(set(self.backend._graph), {
+        self.assertSetEqual(set(self.backend._graph), self.schema_triples | {
             # entity definitions
-            (rdflib.URIRef('http://example.com/me/entity#1234'), rdflib.RDF.type, rdflib.URIRef('http://bsfs.ai/schema/Entity')),
-            (rdflib.URIRef('http://example.com/me/entity#4321'), rdflib.RDF.type, rdflib.URIRef('http://bsfs.ai/schema/Entity')),
+            (rdflib.URIRef('http://example.com/me/entity#1234'), rdflib.RDF.type, rdflib.URIRef('https://schema.bsfs.io/core/Entity')),
+            (rdflib.URIRef('http://example.com/me/entity#4321'), rdflib.RDF.type, rdflib.URIRef('https://schema.bsfs.io/core/Entity')),
             # bookkeeping
             (rdflib.URIRef('http://example.com/me/entity#1234'), rdflib.URIRef(self.t_created.uri), rdflib.Literal(t_ent_created, datatype=rdflib.XSD.integer)),
             (rdflib.URIRef('http://example.com/me/entity#4321'), rdflib.URIRef(self.t_created.uri), rdflib.Literal(t_ent_created, datatype=rdflib.XSD.integer)),
@@ -171,10 +208,10 @@ class TestNodes(unittest.TestCase):
 
         # existing nodes remain unchanged
         self.assertSetEqual(self.ent_ids, nodes._ensure_nodes(self.ent_type, self.ent_ids))
-        self.assertSetEqual(set(self.backend._graph), {
+        self.assertSetEqual(set(self.backend._graph), self.schema_triples | {
             # entity definitions
-            (rdflib.URIRef('http://example.com/me/entity#1234'), rdflib.RDF.type, rdflib.URIRef('http://bsfs.ai/schema/Entity')),
-            (rdflib.URIRef('http://example.com/me/entity#4321'), rdflib.RDF.type, rdflib.URIRef('http://bsfs.ai/schema/Entity')),
+            (rdflib.URIRef('http://example.com/me/entity#1234'), rdflib.RDF.type, rdflib.URIRef('https://schema.bsfs.io/core/Entity')),
+            (rdflib.URIRef('http://example.com/me/entity#4321'), rdflib.RDF.type, rdflib.URIRef('https://schema.bsfs.io/core/Entity')),
             # bookkeeping
             (rdflib.URIRef('http://example.com/me/entity#1234'), rdflib.URIRef(self.t_created.uri), rdflib.Literal(t_ent_created, datatype=rdflib.XSD.integer)),
             (rdflib.URIRef('http://example.com/me/entity#4321'), rdflib.URIRef(self.t_created.uri), rdflib.Literal(t_ent_created, datatype=rdflib.XSD.integer)),
@@ -186,23 +223,23 @@ class TestNodes(unittest.TestCase):
         time_triples = list(self.backend._graph.objects(rdflib.URIRef('http://example.com/me/tag#1234'), rdflib.URIRef(self.t_created.uri)))
         t_tag_created = float(time_triples[0]) if len(time_triples) > 0 else 0.0
         # check triples
-        self.assertSetEqual(set(self.backend._graph), {
+        self.assertSetEqual(set(self.backend._graph), self.schema_triples | {
             # previous triples
-            (rdflib.URIRef('http://example.com/me/entity#1234'), rdflib.RDF.type, rdflib.URIRef('http://bsfs.ai/schema/Entity')),
-            (rdflib.URIRef('http://example.com/me/entity#4321'), rdflib.RDF.type, rdflib.URIRef('http://bsfs.ai/schema/Entity')),
+            (rdflib.URIRef('http://example.com/me/entity#1234'), rdflib.RDF.type, rdflib.URIRef('https://schema.bsfs.io/core/Entity')),
+            (rdflib.URIRef('http://example.com/me/entity#4321'), rdflib.RDF.type, rdflib.URIRef('https://schema.bsfs.io/core/Entity')),
             (rdflib.URIRef('http://example.com/me/entity#1234'), rdflib.URIRef(self.t_created.uri), rdflib.Literal(t_ent_created, datatype=rdflib.XSD.integer)),
             (rdflib.URIRef('http://example.com/me/entity#4321'), rdflib.URIRef(self.t_created.uri), rdflib.Literal(t_ent_created, datatype=rdflib.XSD.integer)),
             # new triples
-            (rdflib.URIRef('http://example.com/me/tag#1234'), rdflib.RDF.type, rdflib.URIRef('http://bsfs.ai/schema/Tag')),
-            (rdflib.URIRef('http://example.com/me/tag#4321'), rdflib.RDF.type, rdflib.URIRef('http://bsfs.ai/schema/Tag')),
+            (rdflib.URIRef('http://example.com/me/tag#1234'), rdflib.RDF.type, rdflib.URIRef('https://schema.bsfs.io/core/Tag')),
+            (rdflib.URIRef('http://example.com/me/tag#4321'), rdflib.RDF.type, rdflib.URIRef('https://schema.bsfs.io/core/Tag')),
             (rdflib.URIRef('http://example.com/me/tag#1234'), rdflib.URIRef(self.t_created.uri), rdflib.Literal(t_tag_created, datatype=rdflib.XSD.integer)),
             (rdflib.URIRef('http://example.com/me/tag#4321'), rdflib.URIRef(self.t_created.uri), rdflib.Literal(t_tag_created, datatype=rdflib.XSD.integer)),
             })
 
     def test___set(self):
         # setup
-        nodes = Nodes(self.backend, self.user, self.ent_type, {URI('http://example.com/me/entity#1234'), URI('http://example.com/me/entity#4321')})
-        self.assertSetEqual(set(self.backend._graph), set())
+        nodes = Nodes(self.backend, self.ac, self.ent_type, {URI('http://example.com/me/entity#1234'), URI('http://example.com/me/entity#4321')})
+        self.assertSetEqual(set(self.backend._graph), self.schema_triples | set())
         set_ = nodes._Nodes__set
 
         # node_type must match predicate's domain
@@ -217,10 +254,10 @@ class TestNodes(unittest.TestCase):
         time_triples = list(self.backend._graph.objects(rdflib.URIRef('http://example.com/me/entity#1234'), rdflib.URIRef(self.t_created.uri)))
         t_ent_created = float(time_triples[0]) if len(time_triples) > 0 else 0.0
         # verify triples
-        self.assertSetEqual(set(self.backend._graph), {
+        self.assertSetEqual(set(self.backend._graph), self.schema_triples | {
             # entity definitions
-            (rdflib.URIRef('http://example.com/me/entity#1234'), rdflib.RDF.type, rdflib.URIRef('http://bsfs.ai/schema/Entity')),
-            (rdflib.URIRef('http://example.com/me/entity#4321'), rdflib.RDF.type, rdflib.URIRef('http://bsfs.ai/schema/Entity')),
+            (rdflib.URIRef('http://example.com/me/entity#1234'), rdflib.RDF.type, rdflib.URIRef('https://schema.bsfs.io/core/Entity')),
+            (rdflib.URIRef('http://example.com/me/entity#4321'), rdflib.RDF.type, rdflib.URIRef('https://schema.bsfs.io/core/Entity')),
             # bookkeeping
             (rdflib.URIRef('http://example.com/me/entity#1234'), rdflib.URIRef(self.t_created.uri), rdflib.Literal(t_ent_created, datatype=rdflib.XSD.integer)),
             (rdflib.URIRef('http://example.com/me/entity#4321'), rdflib.URIRef(self.t_created.uri), rdflib.Literal(t_ent_created, datatype=rdflib.XSD.integer)),
@@ -230,23 +267,23 @@ class TestNodes(unittest.TestCase):
             })
 
         # set node value
-        tags = Nodes(self.backend, self.user, self.tag_type, {URI('http://example.com/me/tag#1234'), URI('http://example.com/me/tag#4321')})
+        tags = Nodes(self.backend, self.ac, self.tag_type, {URI('http://example.com/me/tag#1234'), URI('http://example.com/me/tag#4321')})
         set_(self.p_tag.uri, tags)
         # get creation time from backend manually
         time_triples = list(self.backend._graph.objects(rdflib.URIRef('http://example.com/me/tag#1234'), rdflib.URIRef(self.t_created.uri)))
         t_tag_created = float(time_triples[0]) if len(time_triples) > 0 else 0.0
         # verify triples
-        self.assertSetEqual(set(self.backend._graph), {
+        self.assertSetEqual(set(self.backend._graph), self.schema_triples | {
             # previous values
-            (rdflib.URIRef('http://example.com/me/entity#1234'), rdflib.RDF.type, rdflib.URIRef('http://bsfs.ai/schema/Entity')),
-            (rdflib.URIRef('http://example.com/me/entity#4321'), rdflib.RDF.type, rdflib.URIRef('http://bsfs.ai/schema/Entity')),
+            (rdflib.URIRef('http://example.com/me/entity#1234'), rdflib.RDF.type, rdflib.URIRef('https://schema.bsfs.io/core/Entity')),
+            (rdflib.URIRef('http://example.com/me/entity#4321'), rdflib.RDF.type, rdflib.URIRef('https://schema.bsfs.io/core/Entity')),
             (rdflib.URIRef('http://example.com/me/entity#1234'), rdflib.URIRef(self.p_filesize.uri), rdflib.Literal(1234, datatype=rdflib.XSD.integer)),
             (rdflib.URIRef('http://example.com/me/entity#4321'), rdflib.URIRef(self.p_filesize.uri), rdflib.Literal(1234, datatype=rdflib.XSD.integer)),
             (rdflib.URIRef('http://example.com/me/entity#1234'), rdflib.URIRef(self.t_created.uri), rdflib.Literal(t_ent_created, datatype=rdflib.XSD.integer)),
             (rdflib.URIRef('http://example.com/me/entity#4321'), rdflib.URIRef(self.t_created.uri), rdflib.Literal(t_ent_created, datatype=rdflib.XSD.integer)),
             # tag definitions
-            (rdflib.URIRef('http://example.com/me/tag#1234'), rdflib.RDF.type, rdflib.URIRef('http://bsfs.ai/schema/Tag')),
-            (rdflib.URIRef('http://example.com/me/tag#4321'), rdflib.RDF.type, rdflib.URIRef('http://bsfs.ai/schema/Tag')),
+            (rdflib.URIRef('http://example.com/me/tag#1234'), rdflib.RDF.type, rdflib.URIRef('https://schema.bsfs.io/core/Tag')),
+            (rdflib.URIRef('http://example.com/me/tag#4321'), rdflib.RDF.type, rdflib.URIRef('https://schema.bsfs.io/core/Tag')),
             # tag bookkeeping
             (rdflib.URIRef('http://example.com/me/tag#1234'), rdflib.URIRef(self.t_created.uri), rdflib.Literal(t_tag_created, datatype=rdflib.XSD.integer)),
             (rdflib.URIRef('http://example.com/me/tag#4321'), rdflib.URIRef(self.t_created.uri), rdflib.Literal(t_tag_created, datatype=rdflib.XSD.integer)),
@@ -262,29 +299,29 @@ class TestNodes(unittest.TestCase):
         self.assertRaises(TypeError, set_, self.p_tag.uri, URI('http://example.com/me/tag#1234'))
         # value's node_type must match the predicate's range
         self.assertRaises(errors.ConsistencyError, set_, self.p_tag.uri,
-            Nodes(self.backend, self.user, self.ent_type, self.ent_ids))
+            Nodes(self.backend, self.ac, self.ent_type, self.ent_ids))
 
     def test_set(self):
-        self.assertSetEqual(set(self.backend._graph), set())
-        nodes = Nodes(self.backend, self.user, self.ent_type, self.ent_ids)
+        self.assertSetEqual(set(self.backend._graph), self.schema_triples | set())
+        nodes = Nodes(self.backend, self.ac, self.ent_type, self.ent_ids)
         # can set literal values
         self.assertEqual(nodes, nodes.set(self.p_filesize.uri, 1234))
         self.assertTrue(set(self.backend._graph).issuperset({
             # nodes exist
-            (rdflib.URIRef('http://example.com/me/entity#1234'), rdflib.RDF.type, rdflib.URIRef('http://bsfs.ai/schema/Entity')),
-            (rdflib.URIRef('http://example.com/me/entity#4321'), rdflib.RDF.type, rdflib.URIRef('http://bsfs.ai/schema/Entity')),
+            (rdflib.URIRef('http://example.com/me/entity#1234'), rdflib.RDF.type, rdflib.URIRef('https://schema.bsfs.io/core/Entity')),
+            (rdflib.URIRef('http://example.com/me/entity#4321'), rdflib.RDF.type, rdflib.URIRef('https://schema.bsfs.io/core/Entity')),
             # links exist
             (rdflib.URIRef('http://example.com/me/entity#1234'), rdflib.URIRef(self.p_filesize.uri), rdflib.Literal(1234, datatype=rdflib.XSD.integer)),
             (rdflib.URIRef('http://example.com/me/entity#4321'), rdflib.URIRef(self.p_filesize.uri), rdflib.Literal(1234, datatype=rdflib.XSD.integer)),
             }))
         # can set node values
-        self.assertEqual(nodes, nodes.set(self.p_tag.uri, Nodes(self.backend, self.user, self.tag_type, self.tag_ids)))
+        self.assertEqual(nodes, nodes.set(self.p_tag.uri, Nodes(self.backend, self.ac, self.tag_type, self.tag_ids)))
         self.assertTrue(set(self.backend._graph).issuperset({
             # nodes exist
-            (rdflib.URIRef('http://example.com/me/entity#1234'), rdflib.RDF.type, rdflib.URIRef('http://bsfs.ai/schema/Entity')),
-            (rdflib.URIRef('http://example.com/me/entity#4321'), rdflib.RDF.type, rdflib.URIRef('http://bsfs.ai/schema/Entity')),
-            (rdflib.URIRef('http://example.com/me/tag#1234'), rdflib.RDF.type, rdflib.URIRef('http://bsfs.ai/schema/Tag')),
-            (rdflib.URIRef('http://example.com/me/tag#4321'), rdflib.RDF.type, rdflib.URIRef('http://bsfs.ai/schema/Tag')),
+            (rdflib.URIRef('http://example.com/me/entity#1234'), rdflib.RDF.type, rdflib.URIRef('https://schema.bsfs.io/core/Entity')),
+            (rdflib.URIRef('http://example.com/me/entity#4321'), rdflib.RDF.type, rdflib.URIRef('https://schema.bsfs.io/core/Entity')),
+            (rdflib.URIRef('http://example.com/me/tag#1234'), rdflib.RDF.type, rdflib.URIRef('https://schema.bsfs.io/core/Tag')),
+            (rdflib.URIRef('http://example.com/me/tag#4321'), rdflib.RDF.type, rdflib.URIRef('https://schema.bsfs.io/core/Tag')),
             # links exist
             (rdflib.URIRef('http://example.com/me/entity#1234'), rdflib.URIRef(self.p_tag.uri), rdflib.URIRef('http://example.com/me/tag#1234')),
             (rdflib.URIRef('http://example.com/me/entity#1234'), rdflib.URIRef(self.p_tag.uri), rdflib.URIRef('http://example.com/me/tag#4321')),
@@ -307,24 +344,28 @@ class TestNodes(unittest.TestCase):
         self.assertSetEqual(curr, set(self.backend._graph))
         # cannot assing multiple values to unique predicate
         self.assertRaises(ValueError, nodes.set, self.p_author.uri,
-            Nodes(self.backend, self.user, self.user_type, {URI('http://example.com/me/user#1234'), URI('http://example.com/me/user#4321')}))
+            Nodes(self.backend, self.ac, self.user_type, {URI('http://example.com/me/user#1234'), URI('http://example.com/me/user#4321')}))
         self.assertSetEqual(curr, set(self.backend._graph))
 
+        # can set on empty nodes
+        nodes = Nodes(self.backend, self.ac, self.ent_type, {})
+        self.assertEqual(nodes, nodes.set(self.p_filesize.uri, 1234))
+
 
     def test_set_from_iterable(self):
-        self.assertSetEqual(set(self.backend._graph), set())
-        nodes = Nodes(self.backend, self.user, self.ent_type, self.ent_ids)
+        self.assertSetEqual(set(self.backend._graph), self.schema_triples | set())
+        nodes = Nodes(self.backend, self.ac, self.ent_type, self.ent_ids)
         # can set literal and node values simultaneously
         self.assertEqual(nodes, nodes.set_from_iterable({
             self.p_filesize.uri: 1234,
-            self.p_tag.uri: Nodes(self.backend, self.user, self.tag_type, self.tag_ids),
+            self.p_tag.uri: Nodes(self.backend, self.ac, self.tag_type, self.tag_ids),
             }.items()))
         self.assertTrue(set(self.backend._graph).issuperset({
             # nodes exist
-            (rdflib.URIRef('http://example.com/me/entity#1234'), rdflib.RDF.type, rdflib.URIRef('http://bsfs.ai/schema/Entity')),
-            (rdflib.URIRef('http://example.com/me/entity#4321'), rdflib.RDF.type, rdflib.URIRef('http://bsfs.ai/schema/Entity')),
-            (rdflib.URIRef('http://example.com/me/tag#1234'), rdflib.RDF.type, rdflib.URIRef('http://bsfs.ai/schema/Tag')),
-            (rdflib.URIRef('http://example.com/me/tag#4321'), rdflib.RDF.type, rdflib.URIRef('http://bsfs.ai/schema/Tag')),
+            (rdflib.URIRef('http://example.com/me/entity#1234'), rdflib.RDF.type, rdflib.URIRef('https://schema.bsfs.io/core/Entity')),
+            (rdflib.URIRef('http://example.com/me/entity#4321'), rdflib.RDF.type, rdflib.URIRef('https://schema.bsfs.io/core/Entity')),
+            (rdflib.URIRef('http://example.com/me/tag#1234'), rdflib.RDF.type, rdflib.URIRef('https://schema.bsfs.io/core/Tag')),
+            (rdflib.URIRef('http://example.com/me/tag#4321'), rdflib.RDF.type, rdflib.URIRef('https://schema.bsfs.io/core/Tag')),
             # links exist
             (rdflib.URIRef('http://example.com/me/entity#1234'), rdflib.URIRef(self.p_filesize.uri), rdflib.Literal(1234, datatype=rdflib.XSD.integer)),
             (rdflib.URIRef('http://example.com/me/entity#4321'), rdflib.URIRef(self.p_filesize.uri), rdflib.Literal(1234, datatype=rdflib.XSD.integer)),
@@ -349,9 +390,254 @@ class TestNodes(unittest.TestCase):
         self.assertSetEqual(curr, set(self.backend._graph))
         # cannot assing multiple values to unique predicate
         self.assertRaises(ValueError, nodes.set_from_iterable, ((self.p_filesize.uri, 1234),
-            (self.p_author.uri, Nodes(self.backend, self.user, self.user_type, {URI('http://example.com/me/user#1234'), URI('http://example.com/me/user#4321')}))))
+            (self.p_author.uri, Nodes(self.backend, self.ac, self.user_type, {URI('http://example.com/me/user#1234'), URI('http://example.com/me/user#4321')}))))
         self.assertSetEqual(curr, set(self.backend._graph))
 
+        # can set on empty nodes
+        nodes = Nodes(self.backend, self.ac, self.ent_type, {})
+        self.assertEqual(nodes, nodes.set_from_iterable([(self.p_filesize.uri, 1234)]))
+
+
+    def test_get(self):
+        # setup: add some instances
+        Nodes(self.backend, self.ac, self.ent_type, {'http://example.com/me/entity#1234'}) \
+            .set(ns.bse.comment, 'hello world') \
+            .set(ns.bse.filesize, 1234) \
+            .set(ns.bse.tag, Nodes(self.backend, self.ac, self.tag_type, {'http://example.com/me/tag#1234'}))
+        Nodes(self.backend, self.ac, self.ent_type, {'http://example.com/me/entity#4321'}) \
+            .set(ns.bse.filesize, 4321) \
+            .set(ns.bse.tag, Nodes(self.backend, self.ac, self.tag_type, {'http://example.com/me/tag#4321'}))
+        Nodes(self.backend, self.ac, self.tag_type, {'http://example.com/me/tag#1234'}) \
+            .set(ns.bst.label, 'tag_label_1234')
+        Nodes(self.backend, self.ac, self.tag_type, {'http://example.com/me/tag#4321'}) \
+            .set(ns.bst.label, 'tag_label_4321')
+        # setup: get nodes instance
+        nodes = Nodes(self.backend, self.ac, self.ent_type, self.ent_ids)
+
+        # must pass at least one path
+        self.assertRaises(AttributeError, nodes.get)
+        # view must be list or dict
+        self.assertRaises(ValueError, nodes.get, ns.bse.filesize, view='hello')
+        self.assertRaises(ValueError, nodes.get, ns.bse.filesize, view=1234)
+        self.assertRaises(ValueError, nodes.get, ns.bse.filesize, view=tuple)
+        # can pass path as URI
+        self.assertDictEqual(nodes.get(ns.bse.filesize), {
+            Nodes(self.backend, self.ac, self.ent_type, {'http://example.com/me/entity#1234'}): 1234,
+            Nodes(self.backend, self.ac, self.ent_type, {'http://example.com/me/entity#4321'}): 4321,
+            })
+        # can pass path as sequence of URI
+        self.assertDictEqual(nodes.get((ns.bse.tag, ns.bst.label)), {
+            Nodes(self.backend, self.ac, self.ent_type, {'http://example.com/me/entity#1234'}): {'tag_label_1234'},
+            Nodes(self.backend, self.ac, self.ent_type, {'http://example.com/me/entity#4321'}): {'tag_label_4321'},
+            })
+        # get returns the same path that was passed
+        self.assertCountEqual(list(nodes.get((ns.bse.tag, ns.bst.label), path=True, view=list)), [
+            (Nodes(self.backend, self.ac, self.ent_type, {'http://example.com/me/entity#1234'}), (ns.bse.tag, ns.bst.label), 'tag_label_1234'),
+            (Nodes(self.backend, self.ac, self.ent_type, {'http://example.com/me/entity#4321'}), (ns.bse.tag, ns.bst.label), 'tag_label_4321'),
+            ])
+        self.assertCountEqual(list(nodes.get([ns.bse.tag, ns.bst.label], path=True, view=list)), [
+            (Nodes(self.backend, self.ac, self.ent_type, {'http://example.com/me/entity#1234'}), [ns.bse.tag, ns.bst.label], 'tag_label_1234'),
+            (Nodes(self.backend, self.ac, self.ent_type, {'http://example.com/me/entity#4321'}), [ns.bse.tag, ns.bst.label], 'tag_label_4321'),
+            ])
+        # paths must be URI or sequence thereof
+        self.assertRaises(TypeError, nodes.get, 1234)
+        self.assertRaises(TypeError, nodes.get, (ns.bse.tag, 1234))
+        self.assertRaises(TypeError, nodes.get, (1234, ns.bse.tag))
+        self.assertRaises(ValueError, nodes.get, 'hello world')
+        self.assertRaises(errors.ConsistencyError, nodes.get, 'hello_world')
+        self.assertRaises(errors.ConsistencyError, nodes.get, ns.bse.invalid)
+        self.assertRaises(errors.ConsistencyError, nodes.get, (ns.bse.tag, ns.bst.invalid))
+        # can pass multiple paths
+        self.assertDictEqual(nodes.get(ns.bse.filesize, (ns.bse.tag, ns.bst.label)), {
+            Nodes(self.backend, self.ac, self.ent_type, {'http://example.com/me/entity#1234'}): {
+                ns.bse.filesize: 1234,
+                (ns.bse.tag, ns.bst.label): {'tag_label_1234'},
+                },
+            Nodes(self.backend, self.ac, self.ent_type, {'http://example.com/me/entity#4321'}): {
+                ns.bse.filesize: 4321,
+                (ns.bse.tag, ns.bst.label): {'tag_label_4321'},
+                },
+            })
+        # get respects view
+        self.assertDictEqual(nodes.get(ns.bse.filesize, view=dict), {
+            Nodes(self.backend, self.ac, self.ent_type, {'http://example.com/me/entity#1234'}): 1234,
+            Nodes(self.backend, self.ac, self.ent_type, {'http://example.com/me/entity#4321'}): 4321,
+            })
+        self.assertSetEqual(set(nodes.get(ns.bse.filesize, view=list)), {
+            (Nodes(self.backend, self.ac, self.ent_type, {'http://example.com/me/entity#1234'}), 1234),
+            (Nodes(self.backend, self.ac, self.ent_type, {'http://example.com/me/entity#4321'}), 4321),
+            })
+        # get returns Nodes instance when fetching a node
+        self.assertDictEqual(nodes.get(ns.bse.tag), {
+            Nodes(self.backend, self.ac, self.ent_type, {'http://example.com/me/entity#1234'}):
+                {Nodes(self.backend, self.ac, self.tag_type, {'http://example.com/me/tag#1234'})},
+            Nodes(self.backend, self.ac, self.ent_type, {'http://example.com/me/entity#4321'}):
+                {Nodes(self.backend, self.ac, self.tag_type, {'http://example.com/me/tag#4321'})},
+            })
+        # get returns a value when fetching a value and omits missing values
+        self.assertDictEqual(nodes.get(ns.bse.comment), {
+            Nodes(self.backend, self.ac, self.ent_type, {'http://example.com/me/entity#1234'}): {'hello world'},
+            })
+
+        # results can be empty
+        nodes = Nodes(self.backend, self.ac, self.ent_type, {'http://example.com/me/entity#4321'}) # has filesize, tag but no comment
+        # unique paths return the default value
+        self.assertIsNone(nodes.get(ns.bse.author))
+        self.assertEqual(nodes.get(ns.bse.author, default=1234), 1234)
+        # non-unique paths return an empty set
+        self.assertSetEqual(nodes.get(ns.bse.comment), set())
+
+        # nodes can have no guids
+        nodes = Nodes(self.backend, self.ac, self.ent_type, set())
+        # empty nodes does not excuse an invalid request
+        self.assertRaises(TypeError, nodes.get, 1234)
+        self.assertRaises(errors.ConsistencyError, nodes.get, ns.bse.invalid)
+        # list view always returns an empty list
+        self.assertListEqual(list(nodes.get(ns.bse.comment, view=list)), [])
+        self.assertListEqual(list(nodes.get(ns.bse.comment, view=list, node=True)), [])
+        self.assertListEqual(list(nodes.get(ns.bse.comment, view=list, path=True)), [])
+        self.assertListEqual(list(nodes.get(ns.bse.comment, view=list, node=True, path=True, value=True)), [])
+        self.assertListEqual(list(nodes.get(ns.bse.comment, view=list, node=False)), [])
+        self.assertListEqual(list(nodes.get(ns.bse.comment, view=list, path=False)), [])
+        self.assertListEqual(list(nodes.get(ns.bse.comment, view=list, node=False, path=False, value=False)), [])
+        self.assertListEqual(list(nodes.get(ns.bse.comment, ns.bse.filesize, view=list)), [])
+        self.assertListEqual(list(nodes.get(ns.bse.comment, ns.bse.filesize, view=list, node=True)), [])
+        self.assertListEqual(list(nodes.get(ns.bse.comment, ns.bse.filesize, view=list, path=True)), [])
+        self.assertListEqual(list(nodes.get(ns.bse.comment, ns.bse.filesize, view=list, node=True, path=True, value=True)), [])
+        self.assertListEqual(list(nodes.get(ns.bse.comment, ns.bse.filesize, view=list, node=False)), [])
+        self.assertListEqual(list(nodes.get(ns.bse.comment, ns.bse.filesize, view=list, path=False)), [])
+        self.assertListEqual(list(nodes.get(ns.bse.comment, ns.bse.filesize, view=list, node=False, path=False, value=False)), [])
+        # dict view returns an empty dict or an empty set
+        self.assertDictEqual(nodes.get(ns.bse.comment, view=dict), {})
+        self.assertDictEqual(nodes.get(ns.bse.comment, view=dict, node=True), {})
+        self.assertDictEqual(nodes.get(ns.bse.comment, view=dict, path=True), {})
+        self.assertDictEqual(nodes.get(ns.bse.comment, view=dict, node=True, path=True, value=True, default=None), {})
+        self.assertSetEqual(nodes.get(ns.bse.comment, view=dict, node=False), set())
+        self.assertDictEqual(nodes.get(ns.bse.comment, view=dict, path=False), {})
+        self.assertSetEqual(nodes.get(ns.bse.comment, view=dict, node=False, path=False), set())
+        self.assertSetEqual(nodes.get(ns.bse.comment, view=dict, node=False, path=False, value=False, default=None), set())
+        self.assertDictEqual(nodes.get(ns.bse.comment, ns.bse.filesize, view=dict), {})
+        self.assertDictEqual(nodes.get(ns.bse.comment, ns.bse.filesize, view=dict, node=True), {})
+        self.assertDictEqual(nodes.get(ns.bse.comment, ns.bse.filesize, view=dict, path=True), {})
+        self.assertDictEqual(nodes.get(ns.bse.comment, ns.bse.filesize, view=dict, node=True, path=True, value=True, default=None), {})
+        self.assertDictEqual(nodes.get(ns.bse.comment, ns.bse.filesize, view=dict, node=False), {})
+        self.assertDictEqual(nodes.get(ns.bse.comment, ns.bse.filesize, view=dict, path=False), {})
+        self.assertSetEqual(nodes.get(ns.bse.comment, ns.bse.filesize, view=dict, node=False, path=False), set())
+        self.assertSetEqual(nodes.get(ns.bse.comment, ns.bse.filesize, view=dict, node=False, path=False, value=False, default=None), set())
+
+
+    def test_getattr(self):
+        nodes = Nodes(self.backend, self.ac, self.ent_type, {'http://example.com/me/entity#1234'})
+        # can get walks to values
+        self.assertEqual(nodes.filesize, Walk(nodes, (self.p_filesize, )))
+        # can get walks to nodes
+        self.assertEqual(nodes.tag, Walk(nodes, (self.p_tag, )))
+        # can do multiple hops
+        self.assertEqual(nodes.tag.label, Walk(nodes, (self.p_tag, self.p_label)))
+        # invalid step raises an error
+        self.assertRaises(ValueError, getattr, nodes, 'foobar')
+
+    def test_schema(self):
+        self.assertEqual(Nodes(self.backend, self.ac, self.ent_type,
+            {URI('http://example.com/me/entity#1234')}).schema, self.backend.schema)
+
+    def test_operators(self): # __add__, __or__, __sub__, __and__
+        gen = partial(Nodes, self.backend, self.ac, self.ent_type)
+        nodes = gen({URI('http://example.com/me/entity#1234')})
+        # add/or concatenates guids
+        self.assertEqual(
+            gen({URI('http://example.com/me/entity#1234')}) +
+            gen({URI('http://example.com/me/entity#4321')}),
+            # target
+            gen({
+                URI('http://example.com/me/entity#1234'),
+                URI('http://example.com/me/entity#4321')}))
+        self.assertEqual(
+            gen({URI('http://example.com/me/entity#1234')}) |
+            gen({URI('http://example.com/me/entity#4321')}),
+            # target
+            gen({
+                URI('http://example.com/me/entity#1234'),
+                URI('http://example.com/me/entity#4321')}))
+        # repeated guids are ignored
+        self.assertEqual(
+            gen({URI('http://example.com/me/entity#1234')}) +
+            gen({URI('http://example.com/me/entity#1234')}),
+            # target
+            gen({URI('http://example.com/me/entity#1234')}))
+        self.assertEqual(
+            gen({URI('http://example.com/me/entity#1234')}) |
+            gen({URI('http://example.com/me/entity#1234')}),
+            # target
+            gen({URI('http://example.com/me/entity#1234')}))
+
+        # sub substracts guids
+        self.assertEqual(
+            gen({URI('http://example.com/me/entity#1234'),
+                 URI('http://example.com/me/entity#4321')}) -
+            gen({URI('http://example.com/me/entity#4321')}),
+            # target
+            gen({URI('http://example.com/me/entity#1234')}))
+        # missing guids are ignored
+        self.assertEqual(
+            gen({URI('http://example.com/me/entity#1234')}) -
+            gen({URI('http://example.com/me/entity#4321')}),
+            # target
+            gen({URI('http://example.com/me/entity#1234')}))
+
+        # and intersects guids
+        self.assertEqual(
+            gen({URI('http://example.com/me/entity#1234'),
+                 URI('http://example.com/me/entity#4321')}) &
+            gen({URI('http://example.com/me/entity#4321'),
+                 URI('http://example.com/me/entity#5678')}),
+            # target
+            gen({URI('http://example.com/me/entity#4321')}))
+
+        for op in (operator.add, operator.or_, operator.sub, operator.and_):
+            # type must match
+            self.assertRaises(TypeError, op, nodes, 1234)
+            self.assertRaises(TypeError, op, nodes, 'hello world')
+            # backend must match
+            self.assertRaises(ValueError, op, nodes,
+                Nodes(None, self.ac, self.ent_type, {URI('http://example.com/me/entity#1234')}))
+            # ac must match
+            self.assertRaises(ValueError, op, nodes,
+                Nodes(self.backend, NullAC(self.backend, ''),
+                    self.ent_type, {URI('http://example.com/me/entity#1234')}))
+            # node type must match
+            self.assertRaises(ValueError, op, nodes,
+                Nodes(self.backend, self.ac, self.tag_type, {URI('http://example.com/me/entity#1234')}))
+
+    def test_len(self):
+        self.assertEqual(1, len(Nodes(self.backend, self.ac, self.ent_type, {
+            URI('http://example.com/me/entity#1234'),
+            })))
+        self.assertEqual(2, len(Nodes(self.backend, self.ac, self.ent_type, {
+            URI('http://example.com/me/entity#1234'),
+            URI('http://example.com/me/entity#4321'),
+            })))
+        self.assertEqual(4, len(Nodes(self.backend, self.ac, self.ent_type, {
+            URI('http://example.com/me/entity#1234'),
+            URI('http://example.com/me/entity#4321'),
+            URI('http://example.com/me/entity#5678'),
+            URI('http://example.com/me/entity#8765'),
+            })))
+
+    def test_iter(self): # __iter__
+        gen = partial(Nodes, self.backend, self.ac, self.ent_type)
+        self.assertSetEqual(set(Nodes(self.backend, self.ac, self.ent_type, {
+            URI('http://example.com/me/entity#1234'),
+            URI('http://example.com/me/entity#4321'),
+            URI('http://example.com/me/entity#5678'),
+            URI('http://example.com/me/entity#8765'),
+            })), {
+            gen({URI('http://example.com/me/entity#1234')}),
+            gen({URI('http://example.com/me/entity#4321')}),
+            gen({URI('http://example.com/me/entity#5678')}),
+            gen({URI('http://example.com/me/entity#8765')}),
+            })
+
 
 ## main ##
 
diff --git a/test/graph/test_resolve.py b/test/graph/test_resolve.py
new file mode 100644
index 0000000..e09b1cc
--- /dev/null
+++ b/test/graph/test_resolve.py
@@ -0,0 +1,199 @@
+
+# imports
+import unittest
+
+# bsie imports
+from bsfs import schema as bsc
+from bsfs.graph import Graph, nodes
+from bsfs.namespace import ns
+from bsfs.query import ast
+from bsfs.triple_store import SparqlStore
+from bsfs.utils import URI, errors
+
+# objects to test
+from bsfs.graph.resolve import Filter
+
+
+## code ##
+
+ns.bse = ns.bsfs.Entity()
+
+class TestFilter(unittest.TestCase):
+    """
+
+    NOTE: The Filter resolver is relatively simple as it only checks and changes
+    ast.filter.Is instances. Hence, we don't test all methods individually but
+    all of them with respect to ast.filter.Is elements.
+
+    """
+
+    def test_call(self): # tests resolve implicitly
+        schema = bsc.from_string('''
+            prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
+            prefix xsd: <http://www.w3.org/2001/XMLSchema#>
+
+            prefix bsfs: <https://schema.bsfs.io/core/>
+            prefix bse: <https://schema.bsfs.io/core/Entity#>
+            prefix bsl: <https://schema.bsfs.io/core/Literal/>
+            prefix bsa: <https://schema.bsfs.io/core/Literal/Array/>
+
+            bsfs:Entity rdfs:subClassOf bsfs:Node .
+            bsfs:Tag rdfs:subClassOf bsfs:Node .
+            xsd:string rdfs:subClassOf bsfs:Literal .
+            bsl:Number rdfs:subClassOf bsfs:Literal .
+            bsl:Array rdfs:subClassOf bsfs:Literal .
+            bsa:Feature rdfs:subClassOf bsl:Array .
+            xsd:integer rdfs:subClassOf bsl:Number .
+
+            bsfs:Colors rdfs:subClassOf bsa:Feature ;
+                bsfs:dimension "5"^^xsd:integer .
+
+            bse:colors rdfs:subClassOf bsfs:Predicate ;
+                rdfs:domain bsfs:Entity ;
+                rdfs:range bsfs:Colors .
+
+            bse:comment rdfs:subClassOf bsfs:Predicate ;
+                rdfs:domain bsfs:Entity ;
+                rdfs:range xsd:string ;
+                bsfs:unique "false"^^xsd:boolean .
+
+            bse:filesize rdfs:subClassOf bsfs:Predicate ;
+                rdfs:domain bsfs:Entity ;
+                rdfs:range xsd:integer ;
+                bsfs:unique "false"^^xsd:boolean .
+
+            bse:tag rdfs:subClassOf bsfs:Predicate ;
+                rdfs:domain bsfs:Entity ;
+                rdfs:range bsfs:Tag ;
+                bsfs:unique "false"^^xsd:boolean .
+            ''')
+        backend = SparqlStore.Open()
+        backend.schema = schema
+        graph = Graph(backend, URI('http://example.com/me'))
+        ents = graph.nodes(ns.bsfs.Entity,
+            {URI('http://example.com/me/entity#1234'), URI('http://example.com/me/entity#4321')})
+        tags = graph.nodes(ns.bsfs.Tag,
+            {URI('http://example.com/me/tag#1234'), URI('http://example.com/me/tag#4321')})
+        invalid = nodes.Nodes(None, '', schema.node(ns.bsfs.Node).child(ns.bsfs.Invalid),
+            {'http://example.com/you/invalid#1234', 'http://example.com/you/invalid#4321'})
+        resolver = Filter(schema)
+
+        # query can be None
+        self.assertIsNone(resolver(schema.node(ns.bsfs.Entity), None))
+
+        # immediate Is
+        self.assertEqual(resolver(schema.node(ns.bsfs.Entity),
+            ast.filter.Is(ents)),
+            ast.filter.Or(
+                ast.filter.Is('http://example.com/me/entity#1234'),
+                ast.filter.Is('http://example.com/me/entity#4321')
+                ))
+        # only resolves nodes instances, not URIs
+        self.assertEqual(resolver(schema.node(ns.bsfs.Entity),
+            ast.filter.Is('http://example.com/me/entity#1234')),
+            ast.filter.Is('http://example.com/me/entity#1234'))
+        self.assertEqual(resolver(schema.node(ns.bsfs.Entity),
+            ast.filter.Is(1234)),
+            ast.filter.Is(1234))
+
+        # within And (also checks _value)
+        self.assertEqual(resolver(schema.node(ns.bsfs.Entity),
+            ast.filter.And(
+                ast.filter.Is(ents),
+                ast.filter.Any(ns.bse.comment, ast.filter.Equals('hello world')),
+            )),
+            ast.filter.And(
+                ast.filter.Or(
+                    ast.filter.Is('http://example.com/me/entity#1234'),
+                    ast.filter.Is('http://example.com/me/entity#4321')),
+                ast.filter.Any(ns.bse.comment, ast.filter.Equals('hello world'))
+            ))
+        # within Or (checks _bounded)
+        self.assertEqual(resolver(schema.node(ns.bsfs.Entity),
+            ast.filter.Or(
+                ast.filter.Is(ents),
+                ast.filter.Any(ns.bse.filesize, ast.filter.LessThan(5)),
+            )),
+            ast.filter.Or(
+                ast.filter.Or(
+                    ast.filter.Is('http://example.com/me/entity#1234'),
+                    ast.filter.Is('http://example.com/me/entity#4321')),
+                ast.filter.Any(ns.bse.filesize, ast.filter.LessThan(5))
+            ))
+
+        # Any-branched Is
+        self.assertEqual(resolver(schema.node(ns.bsfs.Entity),
+            ast.filter.Any(ns.bse.tag, ast.filter.Is(tags))),
+            ast.filter.Any(ns.bse.tag, ast.filter.Or(
+                ast.filter.Is('http://example.com/me/tag#1234'),
+                ast.filter.Is('http://example.com/me/tag#4321')),
+                ))
+        # All-branched Is
+        self.assertEqual(resolver(schema.node(ns.bsfs.Entity),
+            ast.filter.All(ns.bse.tag, ast.filter.Is(tags))),
+            ast.filter.All(ns.bse.tag, ast.filter.Or(
+                ast.filter.Is('http://example.com/me/tag#1234'),
+                ast.filter.Is('http://example.com/me/tag#4321')),
+                ))
+        # Negated predicate
+        self.assertEqual(resolver(schema.node(ns.bsfs.Tag),
+            ast.filter.Any(ast.filter.Predicate(ns.bse.tag, reverse=True), ast.filter.Is(ents))),
+            ast.filter.Any(ast.filter.Predicate(ns.bse.tag, reverse=True), ast.filter.Or(
+                ast.filter.Is('http://example.com/me/entity#1234'),
+                ast.filter.Is('http://example.com/me/entity#4321')),
+                ))
+
+        # negated Is
+        self.assertEqual(resolver(schema.node(ns.bsfs.Entity),
+            ast.filter.Not(ast.filter.Is(ents))),
+            ast.filter.Not(
+                ast.filter.Or(
+                    ast.filter.Is('http://example.com/me/entity#1234'),
+                    ast.filter.Is('http://example.com/me/entity#4321')),
+            ))
+
+        # for sake of completeness: Has
+        self.assertEqual(resolver(schema.node(ns.bsfs.Entity),
+            ast.filter.Has(ns.bse.comment)),
+            ast.filter.Has(ns.bse.comment))
+        # for sake of completeness: Distance
+        self.assertEqual(resolver(schema.node(ns.bsfs.Entity),
+            ast.filter.Any(ns.bse.colors, ast.filter.Distance([1,2,3,4,5], 1))),
+            ast.filter.Any(ns.bse.colors, ast.filter.Distance([1,2,3,4,5], 1)))
+        # route errors
+        self.assertRaises(errors.BackendError, resolver, schema.node(ns.bsfs.Tag),
+            ast.filter.Predicate(ns.bse.comment))
+        self.assertRaises(errors.BackendError, resolver, schema.node(ns.bsfs.Tag),
+            ast.filter.Any(ast.filter.PredicateExpression(), ast.filter.Equals('foo')))
+        self.assertRaises(errors.BackendError, resolver._one_of, ast.filter.OneOf(ast.filter.Predicate(ns.bsfs.Predicate)))
+        # for sake of coverage completeness: valid OneOf
+        self.assertIsNotNone(resolver._one_of(ast.filter.OneOf(ast.filter.Predicate(ns.bse.colors))))
+
+        # check schema consistency
+        self.assertRaises(errors.ConsistencyError, resolver, schema.node(ns.bsfs.Tag),
+            ast.filter.Is(invalid))
+        # check immediate type compatibility
+        self.assertRaises(errors.ConsistencyError, resolver, schema.node(ns.bsfs.Tag),
+            ast.filter.Is(ents))
+        self.assertRaises(errors.ConsistencyError, resolver, schema.node(ns.bsfs.Entity),
+            ast.filter.Is(tags))
+        # check type compatibility through branches
+        self.assertRaises(errors.ConsistencyError, resolver, schema.node(ns.bsfs.Tag),
+            ast.filter.Any(ns.bse.comment, ast.filter.Is(tags)))
+        self.assertRaises(errors.ConsistencyError, resolver, schema.node(ns.bsfs.Tag),
+            ast.filter.Any(ns.bse.invalid, ast.filter.Is(tags)))
+        self.assertRaises(errors.ConsistencyError, resolver, schema.node(ns.bsfs.Tag),
+            ast.filter.Any(ast.filter.OneOf(ns.bse.comment, ns.bse.tag), ast.filter.Is(tags)))
+        self.assertRaises(errors.ConsistencyError, resolver, schema.node(ns.bsfs.Tag),
+            ast.filter.Any(ast.filter.OneOf(ns.bse.comment, ns.bse.filesize), ast.filter.Is(tags)))
+        self.assertRaises(errors.ConsistencyError, resolver, schema.node(ns.bsfs.Tag),
+            ast.filter.Any(ast.filter.Predicate(ns.bse.tag, reverse=True), ast.filter.Is(tags)))
+
+
+
+## main ##
+
+if __name__ == '__main__':
+    unittest.main()
+
+## EOF ##
diff --git a/test/graph/test_result.py b/test/graph/test_result.py
new file mode 100644
index 0000000..8960ef6
--- /dev/null
+++ b/test/graph/test_result.py
@@ -0,0 +1,429 @@
+
+# imports
+import unittest
+
+# bsie imports
+from bsfs import schema as bsc
+from bsfs.namespace import ns
+from bsfs.utils import URI
+
+# objects to test
+from bsfs.graph.result import to_list_view, to_dict_view
+
+
+## code ##
+
+ns.bse = ns.bsfs.Entity()
+
+class TestListView(unittest.TestCase):
+    def setUp(self):
+        self.triples_111 = [('ent#1234', ns.bse.iso, 123)]
+        self.triples_11U = [('ent#1234', ns.bse.tag, 'tag#1234'),
+                            ('ent#1234', ns.bse.tag, 'tag#5678')]
+        self.triples_1M1 = [('ent#1234', ns.bse.iso, 123),
+                            ('ent#1234', ns.bse.t_created, '2010-01-02')]
+        self.triples_1MU = [('ent#1234', ns.bse.iso, 123),
+                            ('ent#1234', ns.bse.tag, 'tag#1234'),
+                            ('ent#1234', ns.bse.tag, 'tag#5678')]
+        self.triples_N11 = [('ent#1234', ns.bse.iso, 123),
+                            ('ent#4321', ns.bse.iso, 321)]
+        self.triples_N1U = [('ent#1234', ns.bse.tag, 'tag#1234'),
+                            ('ent#1234', ns.bse.tag, 'tag#5678'),
+                            ('ent#4321', ns.bse.tag, 'tag#4321')]
+        self.triples_NM1 = [('ent#1234', ns.bse.iso, 123),
+                            ('ent#1234', ns.bse.t_created, '2010-01-02'),
+                            ('ent#4321', ns.bse.iso, 321),
+                            ('ent#4321', ns.bse.t_created, '2022-02-22')]
+        self.triples_NMU = [('ent#1234', ns.bse.iso, 123),
+                            ('ent#1234', ns.bse.tag, 'tag#1234'),
+                            ('ent#1234', ns.bse.tag, 'tag#5678'),
+                            ('ent#4321', ns.bse.iso, 321),
+                            ('ent#4321', ns.bse.t_created, '2022-02-22')]
+
+    def test_copy(self):
+        # iterator yields tuples
+        self.assertIsInstance(list(to_list_view([('subject', 'predicate', 'object')], node=True, path=True, value=True))[0], tuple)
+        # components are not changed
+        class Foo(): pass
+        foo = Foo()
+        self.assertListEqual(list(to_list_view([('subject', 'predicate', 'object')], node=True, path=True, value=True)),
+            [('subject', 'predicate', 'object')])
+        self.assertListEqual(list(to_list_view([(foo, 'predicate', 'object')], node=True, path=True, value=True)),
+            [(foo, 'predicate', 'object')])
+        self.assertListEqual(list(to_list_view([('subject', foo, 'object')], node=True, path=True, value=True)),
+            [('subject', foo, 'object')])
+        self.assertListEqual(list(to_list_view([('subject', 'predicate', foo)], node=True, path=True, value=True)),
+            [('subject', 'predicate', foo)])
+
+    def test_agg_none(self):
+        self.assertListEqual(list(to_list_view(self.triples_111, node=True, path=True, value=True)), self.triples_111)
+        self.assertListEqual(list(to_list_view(self.triples_11U, node=True, path=True, value=True)), self.triples_11U)
+        self.assertListEqual(list(to_list_view(self.triples_1M1, node=True, path=True, value=True)), self.triples_1M1)
+        self.assertListEqual(list(to_list_view(self.triples_1MU, node=True, path=True, value=True)), self.triples_1MU)
+        self.assertListEqual(list(to_list_view(self.triples_N11, node=True, path=True, value=True)), self.triples_N11)
+        self.assertListEqual(list(to_list_view(self.triples_N1U, node=True, path=True, value=True)), self.triples_N1U)
+        self.assertListEqual(list(to_list_view(self.triples_NM1, node=True, path=True, value=True)), self.triples_NM1)
+        self.assertListEqual(list(to_list_view(self.triples_NMU, node=True, path=True, value=True)), self.triples_NMU)
+
+    def test_agg_node(self):
+        self.assertListEqual(list(to_list_view(self.triples_111, node=False, path=True, value=True)),
+            [(ns.bse.iso, 123)])
+        self.assertListEqual(list(to_list_view(self.triples_11U, node=False, path=True, value=True)),
+            [(ns.bse.tag, 'tag#1234'), (ns.bse.tag, 'tag#5678')])
+        self.assertListEqual(list(to_list_view(self.triples_1M1, node=False, path=True, value=True)),
+            [(ns.bse.iso, 123), (ns.bse.t_created, '2010-01-02')])
+        self.assertListEqual(list(to_list_view(self.triples_1MU, node=False, path=True, value=True)),
+            [(ns.bse.iso, 123), (ns.bse.tag, 'tag#1234'), (ns.bse.tag, 'tag#5678')])
+        self.assertListEqual(list(to_list_view(self.triples_N11, node=False, path=True, value=True)),
+            [(ns.bse.iso, 123), (ns.bse.iso, 321)])
+        self.assertListEqual(list(to_list_view(self.triples_N1U, node=False, path=True, value=True)),
+            [(ns.bse.tag, 'tag#1234'), (ns.bse.tag, 'tag#5678'), (ns.bse.tag, 'tag#4321')])
+        self.assertListEqual(list(to_list_view(self.triples_NM1, node=False, path=True, value=True)),
+            [(ns.bse.iso, 123), (ns.bse.t_created, '2010-01-02'), (ns.bse.iso, 321), (ns.bse.t_created, '2022-02-22')])
+        self.assertListEqual(list(to_list_view(self.triples_NMU, node=False, path=True, value=True)),
+            [(ns.bse.iso, 123), (ns.bse.tag, 'tag#1234'), (ns.bse.tag, 'tag#5678'), (ns.bse.iso, 321), (ns.bse.t_created, '2022-02-22')])
+
+    def test_agg_path(self):
+        self.assertListEqual(list(to_list_view(self.triples_111, node=True, path=False, value=True)),
+            [('ent#1234', 123)])
+        self.assertListEqual(list(to_list_view(self.triples_11U, node=True, path=False, value=True)),
+            [('ent#1234', 'tag#1234'), ('ent#1234', 'tag#5678')])
+        self.assertListEqual(list(to_list_view(self.triples_1M1, node=True, path=False, value=True)),
+            [('ent#1234', 123), ('ent#1234', '2010-01-02')])
+        self.assertListEqual(list(to_list_view(self.triples_1MU, node=True, path=False, value=True)),
+            [('ent#1234', 123), ('ent#1234', 'tag#1234'), ('ent#1234', 'tag#5678')])
+        self.assertListEqual(list(to_list_view(self.triples_N11, node=True, path=False, value=True)),
+            [('ent#1234', 123), ('ent#4321', 321)])
+        self.assertListEqual(list(to_list_view(self.triples_N1U, node=True, path=False, value=True)),
+            [('ent#1234', 'tag#1234'), ('ent#1234', 'tag#5678'), ('ent#4321', 'tag#4321')])
+        self.assertListEqual(list(to_list_view(self.triples_NM1, node=True, path=False, value=True)),
+            [('ent#1234', 123), ('ent#1234', '2010-01-02'), ('ent#4321', 321), ('ent#4321', '2022-02-22')])
+        self.assertListEqual(list(to_list_view(self.triples_NMU, node=True, path=False, value=True)),
+            [('ent#1234', 123), ('ent#1234', 'tag#1234'), ('ent#1234', 'tag#5678'), ('ent#4321', 321), ('ent#4321', '2022-02-22')])
+
+    def test_agg_node_path(self):
+        self.assertListEqual(list(to_list_view(self.triples_111, node=False, path=False, value=True)),
+            [123])
+        self.assertListEqual(list(to_list_view(self.triples_11U, node=False, path=False, value=True)),
+            ['tag#1234', 'tag#5678'])
+        self.assertListEqual(list(to_list_view(self.triples_1M1, node=False, path=False, value=True)),
+            [123, '2010-01-02'])
+        self.assertListEqual(list(to_list_view(self.triples_1MU, node=False, path=False, value=True)),
+            [123, 'tag#1234', 'tag#5678'])
+        self.assertListEqual(list(to_list_view(self.triples_N11, node=False, path=False, value=True)),
+            [123, 321])
+        self.assertListEqual(list(to_list_view(self.triples_N1U, node=False, path=False, value=True)),
+            ['tag#1234', 'tag#5678', 'tag#4321'])
+        self.assertListEqual(list(to_list_view(self.triples_NM1, node=False, path=False, value=True)),
+            [123, '2010-01-02', 321, '2022-02-22'])
+        self.assertListEqual(list(to_list_view(self.triples_NMU, node=False, path=False, value=True)),
+            [123, 'tag#1234', 'tag#5678', 321, '2022-02-22'])
+
+    def test_agg_value(self):
+        # value flag has no effect
+        self.assertListEqual(list(to_list_view(self.triples_111, node=True, path=True, value=True)), self.triples_111)
+        self.assertListEqual(list(to_list_view(self.triples_11U, node=True, path=True, value=True)), self.triples_11U)
+        self.assertListEqual(list(to_list_view(self.triples_1M1, node=True, path=True, value=True)), self.triples_1M1)
+        self.assertListEqual(list(to_list_view(self.triples_1MU, node=True, path=True, value=True)), self.triples_1MU)
+        self.assertListEqual(list(to_list_view(self.triples_N11, node=True, path=True, value=True)), self.triples_N11)
+        self.assertListEqual(list(to_list_view(self.triples_N1U, node=True, path=True, value=True)), self.triples_N1U)
+        self.assertListEqual(list(to_list_view(self.triples_NM1, node=True, path=True, value=True)), self.triples_NM1)
+        self.assertListEqual(list(to_list_view(self.triples_NMU, node=True, path=True, value=True)), self.triples_NMU)
+
+    def test_agg_node_value(self):
+        # value flag has no effect -> same test as test_agg_node
+        self.assertListEqual(list(to_list_view(self.triples_111, node=False, path=True, value=False)),
+            [(ns.bse.iso, 123)])
+        self.assertListEqual(list(to_list_view(self.triples_11U, node=False, path=True, value=False)),
+            [(ns.bse.tag, 'tag#1234'), (ns.bse.tag, 'tag#5678')])
+        self.assertListEqual(list(to_list_view(self.triples_1M1, node=False, path=True, value=False)),
+            [(ns.bse.iso, 123), (ns.bse.t_created, '2010-01-02')])
+        self.assertListEqual(list(to_list_view(self.triples_1MU, node=False, path=True, value=False)),
+            [(ns.bse.iso, 123), (ns.bse.tag, 'tag#1234'), (ns.bse.tag, 'tag#5678')])
+        self.assertListEqual(list(to_list_view(self.triples_N11, node=False, path=True, value=False)),
+            [(ns.bse.iso, 123), (ns.bse.iso, 321)])
+        self.assertListEqual(list(to_list_view(self.triples_N1U, node=False, path=True, value=False)),
+            [(ns.bse.tag, 'tag#1234'), (ns.bse.tag, 'tag#5678'), (ns.bse.tag, 'tag#4321')])
+        self.assertListEqual(list(to_list_view(self.triples_NM1, node=False, path=True, value=False)),
+            [(ns.bse.iso, 123), (ns.bse.t_created, '2010-01-02'), (ns.bse.iso, 321), (ns.bse.t_created, '2022-02-22')])
+        self.assertListEqual(list(to_list_view(self.triples_NMU, node=False, path=True, value=False)),
+            [(ns.bse.iso, 123), (ns.bse.tag, 'tag#1234'), (ns.bse.tag, 'tag#5678'), (ns.bse.iso, 321), (ns.bse.t_created, '2022-02-22')])
+
+    def test_agg_path_value(self):
+        # value flag has no effect -> same test as test_agg_path
+        self.assertListEqual(list(to_list_view(self.triples_111, node=True, path=False, value=False)),
+            [('ent#1234', 123)])
+        self.assertListEqual(list(to_list_view(self.triples_11U, node=True, path=False, value=False)),
+            [('ent#1234', 'tag#1234'), ('ent#1234', 'tag#5678')])
+        self.assertListEqual(list(to_list_view(self.triples_1M1, node=True, path=False, value=False)),
+            [('ent#1234', 123), ('ent#1234', '2010-01-02')])
+        self.assertListEqual(list(to_list_view(self.triples_1MU, node=True, path=False, value=False)),
+            [('ent#1234', 123), ('ent#1234', 'tag#1234'), ('ent#1234', 'tag#5678')])
+        self.assertListEqual(list(to_list_view(self.triples_N11, node=True, path=False, value=False)),
+            [('ent#1234', 123), ('ent#4321', 321)])
+        self.assertListEqual(list(to_list_view(self.triples_N1U, node=True, path=False, value=False)),
+            [('ent#1234', 'tag#1234'), ('ent#1234', 'tag#5678'), ('ent#4321', 'tag#4321')])
+        self.assertListEqual(list(to_list_view(self.triples_NM1, node=True, path=False, value=False)),
+            [('ent#1234', 123), ('ent#1234', '2010-01-02'), ('ent#4321', 321), ('ent#4321', '2022-02-22')])
+        self.assertListEqual(list(to_list_view(self.triples_NMU, node=True, path=False, value=False)),
+            [('ent#1234', 123), ('ent#1234', 'tag#1234'), ('ent#1234', 'tag#5678'), ('ent#4321', 321), ('ent#4321', '2022-02-22')])
+
+    def test_agg_all(self):
+        # value flag has no effect -> same test as test_agg_node_path
+        self.assertListEqual(list(to_list_view(self.triples_111, node=False, path=False, value=False)),
+            [123])
+        self.assertListEqual(list(to_list_view(self.triples_11U, node=False, path=False, value=False)),
+            ['tag#1234', 'tag#5678'])
+        self.assertListEqual(list(to_list_view(self.triples_1M1, node=False, path=False, value=False)),
+            [123, '2010-01-02'])
+        self.assertListEqual(list(to_list_view(self.triples_1MU, node=False, path=False, value=False)),
+            [123, 'tag#1234', 'tag#5678'])
+        self.assertListEqual(list(to_list_view(self.triples_N11, node=False, path=False, value=False)),
+            [123, 321])
+        self.assertListEqual(list(to_list_view(self.triples_N1U, node=False, path=False, value=False)),
+            ['tag#1234', 'tag#5678', 'tag#4321'])
+        self.assertListEqual(list(to_list_view(self.triples_NM1, node=False, path=False, value=False)),
+            [123, '2010-01-02', 321, '2022-02-22'])
+        self.assertListEqual(list(to_list_view(self.triples_NMU, node=False, path=False, value=False)),
+            [123, 'tag#1234', 'tag#5678', 321, '2022-02-22'])
+
+
+class TestDictView(unittest.TestCase):
+    def setUp(self):
+        self.unique_paths = {ns.bse.iso, ns.bse.t_created}
+        self.triples_111 = [('ent#1234', ns.bse.iso, 123)]
+        self.triples_11U = [('ent#1234', ns.bse.tag, 'tag#1234'),
+                            ('ent#1234', ns.bse.tag, 'tag#5678')]
+        self.triples_1M1 = [('ent#1234', ns.bse.iso, 123),
+                            ('ent#1234', ns.bse.t_created, '2010-01-02')]
+        self.triples_1MU = [('ent#1234', ns.bse.iso, 123),
+                            ('ent#1234', ns.bse.tag, 'tag#1234'),
+                            ('ent#1234', ns.bse.tag, 'tag#5678')]
+        self.triples_N11 = [('ent#1234', ns.bse.iso, 123),
+                            ('ent#4321', ns.bse.iso, 321)]
+        self.triples_N1U = [('ent#1234', ns.bse.tag, 'tag#1234'),
+                            ('ent#1234', ns.bse.tag, 'tag#5678'),
+                            ('ent#4321', ns.bse.tag, 'tag#4321')]
+        self.triples_NM1 = [('ent#1234', ns.bse.iso, 123),
+                            ('ent#1234', ns.bse.t_created, '2010-01-02'),
+                            ('ent#4321', ns.bse.iso, 321),
+                            ('ent#4321', ns.bse.t_created, '2022-02-22')]
+        self.triples_NMU = [('ent#1234', ns.bse.iso, 123),
+                            ('ent#1234', ns.bse.tag, 'tag#1234'),
+                            ('ent#1234', ns.bse.tag, 'tag#5678'),
+                            ('ent#4321', ns.bse.iso, 321),
+                            ('ent#4321', ns.bse.t_created, '2022-02-22')]
+
+    def test_errounous_call(self):
+        # return set instead of value
+        self.assertSetEqual(to_dict_view(self.triples_111, one_node=False, one_path=True,  unique_paths=self.unique_paths, node=False, path=False, value=False),
+            {123})
+        self.assertSetEqual(to_dict_view(self.triples_111, one_node=True,  one_path=False, unique_paths=self.unique_paths, node=False, path=False, value=False),
+            {123})
+        # one_node mismatch: return set of values instead of value
+        self.assertDictEqual(to_dict_view(self.triples_111, one_node=False, one_path=True, unique_paths=self.unique_paths, node=False, path=True, value=False),
+            {ns.bse.iso: {123}})
+        # one_path mismatch: return set of values instead of value
+        self.assertDictEqual(to_dict_view(self.triples_111, one_node=True, one_path=False, unique_paths=self.unique_paths, node=True, path=False, value=False),
+            {'ent#1234': {123}})
+        # unique_paths mismatch: return set of values instead of value
+        self.assertSetEqual(to_dict_view(self.triples_111, one_node=True, one_path=False, unique_paths={}, node=False, path=False, value=False),
+            {123})
+        self.assertDictEqual(to_dict_view(self.triples_111, one_node=True, one_path=False, unique_paths={}, node=False, path=True, value=False),
+            {ns.bse.iso: {123}})
+        self.assertDictEqual(to_dict_view(self.triples_111, one_node=True, one_path=False, unique_paths={}, node=True, path=False, value=False),
+            {'ent#1234': {123}})
+        self.assertDictEqual(to_dict_view(self.triples_111, one_node=True, one_path=False, unique_paths={}, node=True, path=True, value=False),
+            {'ent#1234': {ns.bse.iso: {123}}})
+
+    def test_agg_none(self):
+        self.assertDictEqual(to_dict_view(self.triples_111, one_node=True,  one_path=True,  unique_paths=self.unique_paths, node=True, path=True, value=True),
+            {'ent#1234': {ns.bse.iso: {123}}})
+        self.assertDictEqual(to_dict_view(self.triples_11U, one_node=True,  one_path=True,  unique_paths=self.unique_paths, node=True, path=True, value=True),
+            {'ent#1234': {ns.bse.tag: {'tag#1234', 'tag#5678'}}})
+        self.assertDictEqual(to_dict_view(self.triples_1M1, one_node=True,  one_path=False, unique_paths=self.unique_paths, node=True, path=True, value=True),
+            {'ent#1234': {ns.bse.iso: {123}, ns.bse.t_created: {'2010-01-02'}}})
+        self.assertDictEqual(to_dict_view(self.triples_1MU, one_node=True,  one_path=False, unique_paths=self.unique_paths, node=True, path=True, value=True),
+            {'ent#1234': {ns.bse.iso: {123}, ns.bse.tag: {'tag#1234', 'tag#5678'}}})
+        self.assertDictEqual(to_dict_view(self.triples_N11, one_node=False, one_path=True,  unique_paths=self.unique_paths, node=True, path=True, value=True),
+            {'ent#1234': {ns.bse.iso: {123}}, 'ent#4321': {ns.bse.iso: {321}}})
+        self.assertDictEqual(to_dict_view(self.triples_N1U, one_node=False, one_path=True,  unique_paths=self.unique_paths, node=True, path=True, value=True),
+            {'ent#1234': {ns.bse.tag: {'tag#1234', 'tag#5678'}}, 'ent#4321': {ns.bse.tag: {'tag#4321'}}})
+        self.assertDictEqual(to_dict_view(self.triples_NM1, one_node=False, one_path=False, unique_paths=self.unique_paths, node=True, path=True, value=True),
+            {'ent#1234': {ns.bse.iso: {123}, ns.bse.t_created: {'2010-01-02'}}, 'ent#4321': {ns.bse.iso: {321}, ns.bse.t_created: {'2022-02-22'}}})
+        self.assertDictEqual(to_dict_view(self.triples_NMU, one_node=False, one_path=False, unique_paths=self.unique_paths, node=True, path=True, value=True),
+            {'ent#1234': {ns.bse.iso: {123}, ns.bse.tag: {'tag#1234', 'tag#5678'}}, 'ent#4321': {ns.bse.iso: {321}, ns.bse.t_created: {'2022-02-22'}}})
+        # empty
+        self.assertDictEqual(to_dict_view([], one_node=True,  one_path=True,  unique_paths=self.unique_paths, node=True, path=True, value=True), {})
+        self.assertDictEqual(to_dict_view([], one_node=True,  one_path=False, unique_paths=self.unique_paths, node=True, path=True, value=True), {})
+        self.assertDictEqual(to_dict_view([], one_node=False, one_path=True,  unique_paths=self.unique_paths, node=True, path=True, value=True), {})
+        self.assertDictEqual(to_dict_view([], one_node=False, one_path=False, unique_paths=self.unique_paths, node=True, path=True, value=True), {})
+
+    def test_agg_node(self):
+        self.assertDictEqual(to_dict_view(self.triples_111, one_node=True,  one_path=True,  unique_paths=self.unique_paths, node=False, path=True, value=True),
+            {ns.bse.iso: {123}})
+        self.assertDictEqual(to_dict_view(self.triples_11U, one_node=True,  one_path=True,  unique_paths=self.unique_paths, node=False, path=True, value=True),
+            {ns.bse.tag: {'tag#1234', 'tag#5678'}})
+        self.assertDictEqual(to_dict_view(self.triples_1M1, one_node=True,  one_path=False, unique_paths=self.unique_paths, node=False, path=True, value=True),
+            {ns.bse.iso: {123}, ns.bse.t_created: {'2010-01-02'}})
+        self.assertDictEqual(to_dict_view(self.triples_1MU, one_node=True,  one_path=False, unique_paths=self.unique_paths, node=False, path=True, value=True),
+            {ns.bse.iso: {123}, ns.bse.tag: {'tag#1234', 'tag#5678'}})
+        self.assertDictEqual(to_dict_view(self.triples_N11, one_node=False, one_path=True,  unique_paths=self.unique_paths, node=False, path=True, value=True),
+            {ns.bse.iso: {123, 321}})
+        self.assertDictEqual(to_dict_view(self.triples_N1U, one_node=False, one_path=True,  unique_paths=self.unique_paths, node=False, path=True, value=True),
+            {ns.bse.tag: {'tag#1234', 'tag#5678', 'tag#4321'}})
+        self.assertDictEqual(to_dict_view(self.triples_NM1, one_node=False, one_path=False, unique_paths=self.unique_paths, node=False, path=True, value=True),
+            {ns.bse.iso: {123, 321}, ns.bse.t_created: {'2010-01-02', '2022-02-22'}})
+        self.assertDictEqual(to_dict_view(self.triples_NMU, one_node=False, one_path=False, unique_paths=self.unique_paths, node=False, path=True, value=True),
+            {ns.bse.iso: {123, 321}, ns.bse.tag: {'tag#1234', 'tag#5678'}, ns.bse.t_created: {'2022-02-22'}})
+        # empty
+        self.assertDictEqual(to_dict_view([], one_node=True,  one_path=True,  unique_paths=self.unique_paths, node=False, path=True, value=True), {})
+        self.assertDictEqual(to_dict_view([], one_node=True,  one_path=False, unique_paths=self.unique_paths, node=False, path=True, value=True), {})
+        self.assertDictEqual(to_dict_view([], one_node=False, one_path=True,  unique_paths=self.unique_paths, node=False, path=True, value=True), {})
+        self.assertDictEqual(to_dict_view([], one_node=False, one_path=False, unique_paths=self.unique_paths, node=False, path=True, value=True), {})
+
+    def test_agg_path(self):
+        self.assertDictEqual(to_dict_view(self.triples_111, one_node=True,  one_path=True,  unique_paths=self.unique_paths, node=True, path=False, value=True),
+            {'ent#1234': {123}})
+        self.assertDictEqual(to_dict_view(self.triples_11U, one_node=True,  one_path=True,  unique_paths=self.unique_paths, node=True, path=False, value=True),
+            {'ent#1234': {'tag#1234', 'tag#5678'}})
+        self.assertDictEqual(to_dict_view(self.triples_1M1, one_node=True,  one_path=False, unique_paths=self.unique_paths, node=True, path=False, value=True),
+            {'ent#1234': {123, '2010-01-02'}})
+        self.assertDictEqual(to_dict_view(self.triples_1MU, one_node=True,  one_path=False, unique_paths=self.unique_paths, node=True, path=False, value=True),
+            {'ent#1234': {123, 'tag#1234', 'tag#5678'}})
+        self.assertDictEqual(to_dict_view(self.triples_N11, one_node=False, one_path=True,  unique_paths=self.unique_paths, node=True, path=False, value=True),
+            {'ent#1234': {123}, 'ent#4321': {321}})
+        self.assertDictEqual(to_dict_view(self.triples_N1U, one_node=False, one_path=True,  unique_paths=self.unique_paths, node=True, path=False, value=True),
+            {'ent#1234': {'tag#1234', 'tag#5678'}, 'ent#4321': {'tag#4321'}})
+        self.assertDictEqual(to_dict_view(self.triples_NM1, one_node=False, one_path=False, unique_paths=self.unique_paths, node=True, path=False, value=True),
+            {'ent#1234': {123, '2010-01-02'}, 'ent#4321': {321, '2022-02-22'}})
+        self.assertDictEqual(to_dict_view(self.triples_NMU, one_node=False, one_path=False, unique_paths=self.unique_paths, node=True, path=False, value=True),
+            {'ent#1234': {123, 'tag#1234', 'tag#5678'}, 'ent#4321': {321, '2022-02-22'}})
+        # empty
+        self.assertDictEqual(to_dict_view([], one_node=True,  one_path=True,  unique_paths=self.unique_paths, node=True, path=False, value=True), {})
+        self.assertDictEqual(to_dict_view([], one_node=True,  one_path=False, unique_paths=self.unique_paths, node=True, path=False, value=True), {})
+        self.assertDictEqual(to_dict_view([], one_node=False, one_path=True,  unique_paths=self.unique_paths, node=True, path=False, value=True), {})
+        self.assertDictEqual(to_dict_view([], one_node=False, one_path=False, unique_paths=self.unique_paths, node=True, path=False, value=True), {})
+
+    def test_agg_node_path(self):
+        self.assertSetEqual(to_dict_view(self.triples_111, one_node=True,  one_path=True,  unique_paths=self.unique_paths, node=False, path=False, value=True),
+            {123})
+        self.assertSetEqual(to_dict_view(self.triples_11U, one_node=True,  one_path=True,  unique_paths=self.unique_paths, node=False, path=False, value=True),
+            {'tag#1234', 'tag#5678'})
+        self.assertSetEqual(to_dict_view(self.triples_1M1, one_node=True,  one_path=False, unique_paths=self.unique_paths, node=False, path=False, value=True),
+            {123, '2010-01-02'})
+        self.assertSetEqual(to_dict_view(self.triples_1MU, one_node=True,  one_path=False, unique_paths=self.unique_paths, node=False, path=False, value=True),
+            {123, 'tag#1234', 'tag#5678'})
+        self.assertSetEqual(to_dict_view(self.triples_N11, one_node=False, one_path=True,  unique_paths=self.unique_paths, node=False, path=False, value=True),
+            {123, 321})
+        self.assertSetEqual(to_dict_view(self.triples_N1U, one_node=False, one_path=True,  unique_paths=self.unique_paths, node=False, path=False, value=True),
+            {'tag#1234', 'tag#5678', 'tag#4321'})
+        self.assertSetEqual(to_dict_view(self.triples_NM1, one_node=False, one_path=False, unique_paths=self.unique_paths, node=False, path=False, value=True),
+            {123, '2010-01-02', 321, '2022-02-22'})
+        self.assertSetEqual(to_dict_view(self.triples_NMU, one_node=False, one_path=False, unique_paths=self.unique_paths, node=False, path=False, value=True),
+            {123, 'tag#1234', 'tag#5678', 321, '2022-02-22'})
+        # empty
+        self.assertSetEqual(to_dict_view([], one_node=True,  one_path=True,  unique_paths=self.unique_paths, node=False, path=False, value=True), set())
+        self.assertSetEqual(to_dict_view([], one_node=True,  one_path=False, unique_paths=self.unique_paths, node=False, path=False, value=True), set())
+        self.assertSetEqual(to_dict_view([], one_node=False, one_path=True,  unique_paths=self.unique_paths, node=False, path=False, value=True), set())
+        self.assertSetEqual(to_dict_view([], one_node=False, one_path=False, unique_paths=self.unique_paths, node=False, path=False, value=True), set())
+
+    def test_agg_value(self):
+        self.assertDictEqual(to_dict_view(self.triples_111, one_node=True,  one_path=True,  unique_paths=self.unique_paths, node=True, path=True, value=False),
+            {'ent#1234': {ns.bse.iso: 123}})
+        self.assertDictEqual(to_dict_view(self.triples_11U, one_node=True,  one_path=True,  unique_paths=self.unique_paths, node=True, path=True, value=False),
+            {'ent#1234': {ns.bse.tag: {'tag#1234', 'tag#5678'}}})
+        self.assertDictEqual(to_dict_view(self.triples_1M1, one_node=True,  one_path=False, unique_paths=self.unique_paths, node=True, path=True, value=False),
+            {'ent#1234': {ns.bse.iso: 123, ns.bse.t_created: '2010-01-02'}})
+        self.assertDictEqual(to_dict_view(self.triples_1MU, one_node=True,  one_path=False, unique_paths=self.unique_paths, node=True, path=True, value=False),
+            {'ent#1234': {ns.bse.iso: 123, ns.bse.tag: {'tag#1234', 'tag#5678'}}})
+        self.assertDictEqual(to_dict_view(self.triples_N11, one_node=False, one_path=True,  unique_paths=self.unique_paths, node=True, path=True, value=False),
+            {'ent#1234': {ns.bse.iso: 123}, 'ent#4321': {ns.bse.iso: 321}})
+        self.assertDictEqual(to_dict_view(self.triples_N1U, one_node=False, one_path=True,  unique_paths=self.unique_paths, node=True, path=True, value=False),
+            {'ent#1234': {ns.bse.tag: {'tag#1234', 'tag#5678'}}, 'ent#4321': {ns.bse.tag: {'tag#4321'}}})
+        self.assertDictEqual(to_dict_view(self.triples_NM1, one_node=False, one_path=False, unique_paths=self.unique_paths, node=True, path=True, value=False),
+            {'ent#1234': {ns.bse.iso: 123, ns.bse.t_created: '2010-01-02'}, 'ent#4321': {ns.bse.iso: 321, ns.bse.t_created: '2022-02-22'}})
+        self.assertDictEqual(to_dict_view(self.triples_NMU, one_node=False, one_path=False, unique_paths=self.unique_paths, node=True, path=True, value=False),
+            {'ent#1234': {ns.bse.iso: 123, ns.bse.tag: {'tag#1234', 'tag#5678'}}, 'ent#4321': {ns.bse.iso: 321, ns.bse.t_created: '2022-02-22'}})
+        # empty
+        self.assertDictEqual(to_dict_view([], one_node=True,  one_path=True,  unique_paths=self.unique_paths, node=True, path=True, value=False), {})
+        self.assertDictEqual(to_dict_view([], one_node=True,  one_path=False, unique_paths=self.unique_paths, node=True, path=True, value=False), {})
+        self.assertDictEqual(to_dict_view([], one_node=False, one_path=True,  unique_paths=self.unique_paths, node=True, path=True, value=False), {})
+        self.assertDictEqual(to_dict_view([], one_node=False, one_path=False, unique_paths=self.unique_paths, node=True, path=True, value=False), {})
+
+    def test_agg_node_value(self):
+        self.assertDictEqual(to_dict_view(self.triples_111, one_node=True,  one_path=True,  unique_paths=self.unique_paths, node=False, path=True, value=False),
+            {ns.bse.iso: 123})
+        self.assertDictEqual(to_dict_view(self.triples_11U, one_node=True,  one_path=True,  unique_paths=self.unique_paths, node=False, path=True, value=False),
+            {ns.bse.tag: {'tag#1234', 'tag#5678'}})
+        self.assertDictEqual(to_dict_view(self.triples_1M1, one_node=True,  one_path=False, unique_paths=self.unique_paths, node=False, path=True, value=False),
+            {ns.bse.iso: 123, ns.bse.t_created: '2010-01-02'})
+        self.assertDictEqual(to_dict_view(self.triples_1MU, one_node=True,  one_path=False, unique_paths=self.unique_paths, node=False, path=True, value=False),
+            {ns.bse.iso: 123, ns.bse.tag: {'tag#1234', 'tag#5678'}})
+        self.assertDictEqual(to_dict_view(self.triples_N11, one_node=False, one_path=True,  unique_paths=self.unique_paths, node=False, path=True, value=False),
+            {ns.bse.iso: {123, 321}})
+        self.assertDictEqual(to_dict_view(self.triples_N1U, one_node=False, one_path=True,  unique_paths=self.unique_paths, node=False, path=True, value=False),
+            {ns.bse.tag: {'tag#1234', 'tag#5678', 'tag#4321'}})
+        self.assertDictEqual(to_dict_view(self.triples_NM1, one_node=False, one_path=False, unique_paths=self.unique_paths, node=False, path=True, value=False),
+            {ns.bse.iso: {123, 321}, ns.bse.t_created: {'2010-01-02', '2022-02-22'}})
+        self.assertDictEqual(to_dict_view(self.triples_NMU, one_node=False, one_path=False, unique_paths=self.unique_paths, node=False, path=True, value=False),
+            {ns.bse.iso: {123, 321}, ns.bse.tag: {'tag#1234', 'tag#5678'}, ns.bse.t_created: {'2022-02-22'}})
+        # empty
+        self.assertDictEqual(to_dict_view([], one_node=True,  one_path=True,  unique_paths=self.unique_paths, node=False, path=True, value=False), {})
+        self.assertDictEqual(to_dict_view([], one_node=True,  one_path=False, unique_paths=self.unique_paths, node=False, path=True, value=False), {})
+        self.assertDictEqual(to_dict_view([], one_node=False, one_path=True,  unique_paths=self.unique_paths, node=False, path=True, value=False), {})
+        self.assertDictEqual(to_dict_view([], one_node=False, one_path=False, unique_paths=self.unique_paths, node=False, path=True, value=False), {})
+
+    def test_agg_path_value(self):
+        self.assertDictEqual(to_dict_view(self.triples_111, one_node=True,  one_path=True,  unique_paths=self.unique_paths, node=True, path=False, value=False),
+            {'ent#1234': 123})
+        self.assertDictEqual(to_dict_view(self.triples_11U, one_node=True,  one_path=True,  unique_paths=self.unique_paths, node=True, path=False, value=False),
+            {'ent#1234': {'tag#1234', 'tag#5678'}})
+        self.assertDictEqual(to_dict_view(self.triples_1M1, one_node=True,  one_path=False, unique_paths=self.unique_paths, node=True, path=False, value=False),
+            {'ent#1234': {123, '2010-01-02'}})
+        self.assertDictEqual(to_dict_view(self.triples_1MU, one_node=True,  one_path=False, unique_paths=self.unique_paths, node=True, path=False, value=False),
+            {'ent#1234': {123, 'tag#1234', 'tag#5678'}})
+        self.assertDictEqual(to_dict_view(self.triples_N11, one_node=False, one_path=True,  unique_paths=self.unique_paths, node=True, path=False, value=False),
+            {'ent#1234': 123, 'ent#4321': 321})
+        self.assertDictEqual(to_dict_view(self.triples_N1U, one_node=False, one_path=True,  unique_paths=self.unique_paths, node=True, path=False, value=False),
+            {'ent#1234': {'tag#1234', 'tag#5678'}, 'ent#4321': {'tag#4321'}})
+        self.assertDictEqual(to_dict_view(self.triples_NM1, one_node=False, one_path=False, unique_paths=self.unique_paths, node=True, path=False, value=False),
+            {'ent#1234': {123, '2010-01-02'}, 'ent#4321': {321, '2022-02-22'}})
+        self.assertDictEqual(to_dict_view(self.triples_NMU, one_node=False, one_path=False, unique_paths=self.unique_paths, node=True, path=False, value=False),
+            {'ent#1234': {123, 'tag#1234', 'tag#5678'}, 'ent#4321': {321, '2022-02-22'}})
+        # empty
+        self.assertDictEqual(to_dict_view([], one_node=True,  one_path=True,  unique_paths=self.unique_paths, node=True, path=False, value=False), {})
+        self.assertDictEqual(to_dict_view([], one_node=True,  one_path=False, unique_paths=self.unique_paths, node=True, path=False, value=False), {})
+        self.assertDictEqual(to_dict_view([], one_node=False, one_path=True,  unique_paths=self.unique_paths, node=True, path=False, value=False), {})
+        self.assertDictEqual(to_dict_view([], one_node=False, one_path=False, unique_paths=self.unique_paths, node=True, path=False, value=False), {})
+
+    def test_agg_all(self):
+        self.assertEqual(to_dict_view(self.triples_111, one_node=True,  one_path=True,  unique_paths=self.unique_paths, node=False, path=False, value=False),
+            123)
+        self.assertSetEqual(to_dict_view(self.triples_11U, one_node=True,  one_path=True,  unique_paths=self.unique_paths, node=False, path=False, value=False),
+            {'tag#1234', 'tag#5678'})
+        self.assertSetEqual(to_dict_view(self.triples_1M1, one_node=True,  one_path=False, unique_paths=self.unique_paths, node=False, path=False, value=False),
+            {123, '2010-01-02'})
+        self.assertSetEqual(to_dict_view(self.triples_1MU, one_node=True,  one_path=False, unique_paths=self.unique_paths, node=False, path=False, value=False),
+            {123, 'tag#1234', 'tag#5678'})
+        self.assertSetEqual(to_dict_view(self.triples_N11, one_node=False, one_path=True,  unique_paths=self.unique_paths, node=False, path=False, value=False),
+            {123, 321})
+        self.assertSetEqual(to_dict_view(self.triples_N1U, one_node=False, one_path=True,  unique_paths=self.unique_paths, node=False, path=False, value=False),
+            {'tag#1234', 'tag#5678', 'tag#4321'})
+        self.assertSetEqual(to_dict_view(self.triples_NM1, one_node=False, one_path=False, unique_paths=self.unique_paths, node=False, path=False, value=False),
+            {123, '2010-01-02', 321, '2022-02-22'})
+        self.assertSetEqual(to_dict_view(self.triples_NMU, one_node=False, one_path=False, unique_paths=self.unique_paths, node=False, path=False, value=False),
+            {123, 'tag#1234', 'tag#5678', 321, '2022-02-22'})
+        # empty
+        self.assertEqual(to_dict_view([], one_node=True,  one_path=True,  unique_paths=self.unique_paths, node=False, path=False, value=False), None)
+        self.assertSetEqual(to_dict_view([], one_node=True,  one_path=False, unique_paths=self.unique_paths, node=False, path=False, value=False), set())
+        self.assertSetEqual(to_dict_view([], one_node=False, one_path=True,  unique_paths=self.unique_paths, node=False, path=False, value=False), set())
+        self.assertSetEqual(to_dict_view([], one_node=False, one_path=False, unique_paths=self.unique_paths, node=False, path=False, value=False), set())
+        self.assertEqual(to_dict_view([], one_node=True,  one_path=True,  unique_paths=self.unique_paths, node=False, path=False, value=False, default=123), 123)
+
+## main ##
+
+if __name__ == '__main__':
+    unittest.main()
+
+## EOF ##
diff --git a/test/graph/test_walk.py b/test/graph/test_walk.py
new file mode 100644
index 0000000..4b844da
--- /dev/null
+++ b/test/graph/test_walk.py
@@ -0,0 +1,170 @@
+
+# imports
+import unittest
+
+# bsfs imports
+from bsfs import schema as bsc
+from bsfs.graph import Graph
+from bsfs.graph.ac import NullAC
+from bsfs.namespace import Namespace, ns
+from bsfs.triple_store.sparql import SparqlStore
+from bsfs.utils import URI
+
+# symbol to test
+from bsfs.graph.walk import Walk
+
+## code ##
+
+ns.bse = ns.bsfs.Entity()
+ns.bst = ns.bsfs.Tag()
+
+class TestWalk(unittest.TestCase):
+    def setUp(self):
+        # backend setup
+        self.schema = bsc.from_string('''
+            prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
+            prefix xsd: <http://www.w3.org/2001/XMLSchema#>
+            prefix bsfs: <https://schema.bsfs.io/core/>
+            prefix bse: <https://schema.bsfs.io/core/Entity#>
+            prefix bst: <https://schema.bsfs.io/core/Tag#>
+
+            bsfs:Entity rdfs:subClassOf bsfs:Node .
+            bsfs:Tag rdfs:subClassOf bsfs:Node .
+            bsfs:User rdfs:subClassOf bsfs:Node .
+            xsd:string rdfs:subClassOf bsfs:Literal .
+
+            bse:author rdfs:subClassOf bsfs:Predicate ;
+                rdfs:domain bsfs:Node ;
+                rdfs:range bsfs:User .
+
+            bse:tag rdfs:subClassOf bsfs:Predicate ;
+                rdfs:domain bsfs:Entity ;
+                rdfs:range bsfs:Tag .
+
+            bst:label rdfs:subClassOf bsfs:Predicate ;
+                rdfs:domain bsfs:Tag ;
+                rdfs:range xsd:string ;
+                bsfs:unique "true"^^xsd:boolean .
+
+            bst:subTagOf rdfs:subClassOf bsfs:Predicate ;
+                rdfs:domain bsfs:Tag ;
+                rdfs:range bsfs:Tag .
+
+            bst:main rdfs:subClassOf bsfs:Predicate ;
+                rdfs:domain bsfs:Tag ;
+                rdfs:range bsfs:Entity .
+
+            bst:author rdfs:subClassOf bsfs:Predicate ;
+                rdfs:domain bsfs:Tag ;
+                rdfs:range xsd:string .
+
+            ''')
+        self.backend = SparqlStore.Open()
+        self.user = URI('http://example.com/me')
+        self.ac = NullAC(self.backend, self.user)
+        self.graph = Graph(self.backend, self.ac)
+        self.graph.migrate(self.schema)
+
+        # nodes setup
+        self.ents = self.graph.nodes(ns.bsfs.Entity, {
+            URI('http://example.com/me/entity#1234'),
+            URI('http://example.com/me/entity#4321')})
+        self.tags = self.graph.nodes(ns.bsfs.Tag, {
+            URI('http://example.com/me/tag#1234'),
+            URI('http://example.com/me/tag#4321')})
+        # add some instances
+        self.ents.set(ns.bse.tag, self.tags)
+        self.graph.node(ns.bsfs.Tag, URI('http://example.com/me/tag#1234')).set(ns.bst.label, 'hello')
+        self.graph.node(ns.bsfs.Tag, URI('http://example.com/me/tag#4321')).set(ns.bst.label, 'world')
+
+    def test_essentials(self): # __eq__, __hash__, __str__, __repr__
+        p_author = self.schema.predicate(ns.bse.author)
+        p_tag = self.schema.predicate(ns.bse.tag)
+        p_main = self.schema.predicate(ns.bst.main)
+        # comparison
+        self.assertEqual(Walk(self.ents, [p_tag]), Walk(self.ents, [p_tag]))
+        self.assertEqual(hash(Walk(self.ents, [p_tag])), hash(Walk(self.ents, [p_tag])))
+        # comparison respects type
+        class Foo(Walk): pass
+        self.assertNotEqual(Walk(self.ents, [p_tag]), Foo(self.ents, [p_tag]))
+        self.assertNotEqual(hash(Walk(self.ents, [p_tag])), hash(Foo(self.ents, [p_tag])))
+        # comparison respects root
+        self.assertNotEqual(Walk(self.ents, [p_author]), Walk(self.tags, [p_author]))
+        self.assertNotEqual(hash(Walk(self.ents, [p_author])), hash(Walk(self.tags, [p_author])))
+        # comparison respects path
+        self.assertNotEqual(Walk(self.tags, [p_author]), Walk(self.tags, [p_main]))
+        self.assertNotEqual(hash(Walk(self.tags, [p_author])), hash(Walk(self.tags, [p_main])))
+        # string conversion
+        self.assertEqual(str(Walk(self.ents, [p_tag, p_main])),
+            'Walk(@https://schema.bsfs.io/core/Entity: https://schema.bsfs.io/core/Entity#tag, https://schema.bsfs.io/core/Tag#main)')
+        self.assertEqual(repr(Walk(self.ents, [p_tag, p_main])),
+            'Walk(https://schema.bsfs.io/core/Entity, (https://schema.bsfs.io/core/Entity#tag, https://schema.bsfs.io/core/Tag#main))')
+
+    def test_tail(self):
+        self.assertEqual(Walk(self.ents, (
+            self.schema.predicate(ns.bse.tag),
+            )).tail,
+            self.schema.node(ns.bsfs.Tag))
+        self.assertEqual(Walk(self.ents, (
+            self.schema.predicate(ns.bse.tag),
+            self.schema.predicate(ns.bst.main),
+            )).tail,
+            self.schema.node(ns.bsfs.Entity))
+
+    def test_step(self):
+        tag_type = self.schema.node(ns.bsfs.Tag)
+        # step returns a predicate
+        self.assertEqual(Walk.step(self.schema, tag_type, 'subTagOf'),
+            (self.schema.predicate(ns.bst.subTagOf), ))
+        # invalid step raises an error
+        self.assertRaises(ValueError, Walk.step, self.schema, tag_type, 'foobar')
+        # ambiguous step raises an error
+        self.assertRaises(ValueError, Walk.step, self.schema, tag_type, 'author')
+
+    def test_getattr(self): # __getattr__
+        walk = Walk(self.ents, (self.schema.predicate(ns.bse.tag), ))
+        # first step
+        self.assertEqual(walk.subTagOf, Walk(self.ents, (
+            self.schema.predicate(ns.bse.tag),
+            self.schema.predicate(ns.bst.subTagOf),
+            )))
+        # second step
+        self.assertEqual(walk.subTagOf.main, Walk(self.ents, (
+            self.schema.predicate(ns.bse.tag),
+            self.schema.predicate(ns.bst.subTagOf),
+            self.schema.predicate(ns.bst.main),
+            )))
+        # invalid step raises an error
+        self.assertRaises(ValueError, getattr, walk, 'foobar')
+        # ambiguous step raises an error
+        self.assertRaises(ValueError, getattr, walk, 'author')
+
+    def test_get(self): # get, __call__
+        walk = Walk(self.ents, (self.schema.predicate(ns.bse.tag), ))
+        tags = {
+            self.graph.node(ns.bsfs.Tag, URI('http://example.com/me/tag#1234')),
+            self.graph.node(ns.bsfs.Tag, URI('http://example.com/me/tag#4321'))}
+        # get returns from Nodes.get
+        self.assertDictEqual(walk.get(), {
+            self.graph.node(ns.bsfs.Entity, URI('http://example.com/me/entity#1234')): tags,
+            self.graph.node(ns.bsfs.Entity, URI('http://example.com/me/entity#4321')): tags,
+            })
+        self.assertDictEqual(walk(), {
+            self.graph.node(ns.bsfs.Entity, URI('http://example.com/me/entity#1234')): tags,
+            self.graph.node(ns.bsfs.Entity, URI('http://example.com/me/entity#4321')): tags,
+            })
+        # get passes kwargs to Nodes.get
+        self.assertSetEqual(tags, walk.get(node=False))
+        self.assertSetEqual(tags, walk(node=False))
+        self.assertSetEqual(tags, set(walk.get(view=list, node=False)))
+        self.assertSetEqual(tags, set(walk(view=list, node=False)))
+        # get returns values if need be
+        self.assertSetEqual(walk.label(node=False), {'hello', 'world'})
+
+
+## main ##
+
+if __name__ == '__main__':
+    unittest.main()
+
+## EOF ##
diff --git a/test/namespace/test_namespace.py b/test/namespace/test_namespace.py
index f109653..f7bf02a 100644
--- a/test/namespace/test_namespace.py
+++ b/test/namespace/test_namespace.py
@@ -1,9 +1,4 @@
-"""
 
-Part of the tagit test suite.
-A copy of the license is provided with the project.
-Author: Matthias Baumgartner, 2022
-"""
 # imports
 import operator
 import unittest
@@ -12,7 +7,7 @@ import unittest
 from bsfs.utils import URI
 
 # objects to test
-from bsfs.namespace.namespace import Namespace, ClosedNamespace
+from bsfs.namespace.namespace import Namespace, FinalNamespace
 
 
 ## code ##
@@ -20,108 +15,48 @@ from bsfs.namespace.namespace import Namespace, ClosedNamespace
 class TestNamespace(unittest.TestCase):
     def test_essentials(self):
         # string conversion
-        self.assertEqual(str(Namespace('http://example.org/')), 'Namespace(http://example.org)')
-        self.assertEqual(str(Namespace('http://example.org#')), 'Namespace(http://example.org)')
-        self.assertEqual(repr(Namespace('http://example.org/')), 'Namespace(http://example.org, #, /)')
-        self.assertEqual(repr(Namespace('http://example.org#')), 'Namespace(http://example.org, #, /)')
-        self.assertEqual(repr(Namespace('http://example.org', fsep='.')), 'Namespace(http://example.org, ., /)')
-        self.assertEqual(repr(Namespace('http://example.org', psep='.')), 'Namespace(http://example.org, #, .)')
-        # repeated separators are truncated
-        self.assertEqual(str(Namespace('http://example.org////')), 'Namespace(http://example.org)')
-        self.assertEqual(str(Namespace('http://example.org####')), 'Namespace(http://example.org)')
-        self.assertEqual(repr(Namespace('http://example.org///##')), 'Namespace(http://example.org, #, /)')
+        self.assertEqual(str(Namespace('http://example.org')), 'http://example.org')
+        self.assertEqual(repr(Namespace('http://example.org')), "'http://example.org'")
         # comparison
-        class Foo(Namespace): pass
-        self.assertEqual(Namespace('http://example.org/'), Namespace('http://example.org/'))
-        self.assertEqual(Namespace('http://example.org/'), Namespace('http://example.org'))
-        self.assertEqual(Namespace('http://example.org/'), Namespace('http://example.org#'))
-        self.assertNotEqual(Namespace('http://example.org'), Namespace('http://example.org', fsep='.'))
-        self.assertNotEqual(Namespace('http://example.org'), Namespace('http://example.org', psep='.'))
-        self.assertNotEqual(Namespace('http://example.org/'), Foo('http://example.org/'))
-        self.assertNotEqual(Foo('http://example.org/'), Namespace('http://example.org/'))
-        # hashing
-        self.assertEqual(hash(Namespace('http://example.org/')), hash(Namespace('http://example.org/')))
-        self.assertEqual(hash(Namespace('http://example.org/')), hash(Namespace('http://example.org')))
-        self.assertEqual(hash(Namespace('http://example.org/')), hash(Namespace('http://example.org#')))
+        self.assertEqual(Namespace('http://example.org'), Namespace('http://example.org'))
+        self.assertEqual(hash(Namespace('http://example.org')), hash(Namespace('http://example.org')))
+        # Namespace compares to string
+        self.assertEqual(Namespace('http://example.org'), 'http://example.org')
+        self.assertEqual(hash(Namespace('http://example.org')), hash('http://example.org'))
+        # URI must match
+        self.assertNotEqual(Namespace('http://example.org'), Namespace('http://example.com'))
         self.assertNotEqual(hash(Namespace('http://example.org')), hash(Namespace('http://example.com')))
-        self.assertNotEqual(hash(Namespace('http://example.org')), hash(Namespace('http://example.org', fsep='.')))
-        self.assertNotEqual(hash(Namespace('http://example.org')), hash(Namespace('http://example.org', psep='.')))
-        self.assertNotEqual(hash(Namespace('http://example.org/')), hash(Foo('http://example.org/')))
-        self.assertNotEqual(hash(Foo('http://example.org/')), hash(Namespace('http://example.org/')))
 
     def test_getattr(self):
-        self.assertEqual(Namespace('http://example.org/').foo, 'http://example.org#foo')
-        self.assertEqual(Namespace('http://example.org/').bar, 'http://example.org#bar')
-        self.assertEqual(Namespace('http://example.org/', fsep='/').foo, 'http://example.org/foo')
-        self.assertEqual(Namespace('http://example.org/', fsep='/').bar, 'http://example.org/bar')
-        self.assertEqual(Namespace('http://example.org', fsep='/').foo, 'http://example.org/foo')
-        self.assertEqual(Namespace('http://example.org', fsep='/').bar, 'http://example.org/bar')
-        self.assertEqual(Namespace('http://example.org#', fsep='/').foo, 'http://example.org#/foo')
-        self.assertEqual(Namespace('http://example.org#', fsep='/').bar, 'http://example.org#/bar')
-        self.assertEqual(Namespace('http://example.org/me#').foo, 'http://example.org/me#foo')
-        self.assertEqual(Namespace('http://example.org/me#').bar, 'http://example.org/me#bar')
+        self.assertEqual(Namespace('http://example.org').foo, Namespace('http://example.org/foo'))
+        self.assertEqual(Namespace('http://example.org').bar, Namespace('http://example.org/bar'))
 
-    def test_getitem(self):
-        self.assertEqual(Namespace('http://example.org')['foo'], 'http://example.org#foo')
-        self.assertEqual(Namespace('http://example.org')['bar'], 'http://example.org#bar')
-        self.assertEqual(Namespace('http://example.org', fsep='/')['foo'], 'http://example.org/foo')
-        self.assertEqual(Namespace('http://example.org', fsep='/')['bar'], 'http://example.org/bar')
-        self.assertEqual(Namespace('http://example.org/me#')['foo'], 'http://example.org/me#foo')
-        self.assertEqual(Namespace('http://example.org/me#')['bar'], 'http://example.org/me#bar')
+    def test_call(self):
+        self.assertEqual(Namespace('http://example.org')(), FinalNamespace('http://example.org', sep='#'))
+        self.assertEqual(Namespace('http://example.org').foo(), FinalNamespace('http://example.org/foo', sep='#'))
 
-    def test_add(self):
-        self.assertEqual(Namespace('http://example.org') + 'foo', Namespace('http://example.org/foo'))
-        self.assertEqual(Namespace('http://example.org', psep='.') + 'foo', Namespace('http://example.org.foo', psep='.'))
-        self.assertEqual(Namespace('http://example.org') + 'foo' + 'bar', Namespace('http://example.org/foo/bar'))
-        # can add URIs
-        self.assertEqual(Namespace('http://example.org') + URI('foo'), Namespace('http://example.org/foo'))
-        # can only add strings
-        self.assertRaises(TypeError, operator.add, Namespace('http://example.org'), 1234)
-        self.assertRaises(TypeError, operator.add, Namespace('http://example.org'), Namespace('http://example.com'))
-
-
-class TestClosedNamespace(unittest.TestCase):
+class TestFinalNamespace(unittest.TestCase):
     def test_essentials(self):
-       # string conversion
-        self.assertEqual(str(ClosedNamespace('http://example.org/')), 'ClosedNamespace(http://example.org)')
-        self.assertEqual(str(ClosedNamespace('http://example.org#')), 'ClosedNamespace(http://example.org)')
-        self.assertEqual(repr(ClosedNamespace('http://example.org/')), 'ClosedNamespace(http://example.org, #, /)')
-        self.assertEqual(repr(ClosedNamespace('http://example.org#')), 'ClosedNamespace(http://example.org, #, /)')
-        self.assertEqual(repr(ClosedNamespace('http://example.org', fsep='.')), 'ClosedNamespace(http://example.org, ., /)')
-        self.assertEqual(repr(ClosedNamespace('http://example.org', psep='.')), 'ClosedNamespace(http://example.org, #, .)')
+        # string conversion
+        self.assertEqual(str(FinalNamespace('http://example.org')), 'http://example.org')
+        self.assertEqual(repr(FinalNamespace('http://example.org')), "'http://example.org'")
         # comparison
-        class Foo(ClosedNamespace): pass
-        self.assertEqual(ClosedNamespace('http://example.org'), ClosedNamespace('http://example.org#'))
-        self.assertEqual(ClosedNamespace('http://example.org'), ClosedNamespace('http://example.org'))
-        self.assertEqual(ClosedNamespace('http://example.org'), ClosedNamespace('http://example.org/'))
-        self.assertEqual(ClosedNamespace('http://example.org/', 'foo', 'bar'), ClosedNamespace('http://example.org/', 'foo', 'bar'))
-        self.assertNotEqual(ClosedNamespace('http://example.org/', 'foo'), ClosedNamespace('http://example.org/', 'bar'))
-        self.assertNotEqual(ClosedNamespace('http://example.org/'), Foo('http://example.org/'))
-        self.assertNotEqual(Foo('http://example.org/'), ClosedNamespace('http://example.org/'))
-        # hashing
-        self.assertEqual(hash(ClosedNamespace('http://example.org')), hash(ClosedNamespace('http://example.org')))
-        self.assertEqual(hash(ClosedNamespace('http://example.org')), hash(ClosedNamespace('http://example.org/')))
-        self.assertEqual(hash(ClosedNamespace('http://example.org')), hash(ClosedNamespace('http://example.org#')))
-        self.assertEqual(hash(ClosedNamespace('http://example.org/', 'foo', 'bar')), hash(ClosedNamespace('http://example.org/', 'foo', 'bar')))
-        self.assertNotEqual(hash(ClosedNamespace('http://example.org/', 'foo')), hash(ClosedNamespace('http://example.org/', 'bar')))
-        self.assertNotEqual(hash(ClosedNamespace('http://example.org/')), hash(Foo('http://example.org/')))
-        self.assertNotEqual(hash(Foo('http://example.org/')), hash(ClosedNamespace('http://example.org/')))
+        self.assertEqual(FinalNamespace('http://example.org'), FinalNamespace('http://example.org'))
+        self.assertEqual(hash(FinalNamespace('http://example.org')), hash(FinalNamespace('http://example.org')))
+        # FinalNamespace compares to string
+        self.assertEqual(FinalNamespace('http://example.org'), 'http://example.org')
+        self.assertEqual(hash(FinalNamespace('http://example.org')), hash('http://example.org'))
+        # URI must match
+        self.assertNotEqual(FinalNamespace('http://example.org'), FinalNamespace('http://example.com'))
+        self.assertNotEqual(hash(FinalNamespace('http://example.org')), hash(FinalNamespace('http://example.com')))
+        # separator is ignored
+        self.assertEqual(FinalNamespace('http://example.org'), FinalNamespace('http://example.org', sep='/'))
+        self.assertEqual(hash(FinalNamespace('http://example.org')), hash(FinalNamespace('http://example.org', sep='/')))
 
     def test_getattr(self):
-        self.assertEqual(ClosedNamespace('http://example.org/', 'foo', 'bar').foo, 'http://example.org#foo')
-        self.assertEqual(ClosedNamespace('http://example.org/', 'bar', 'bar').bar, 'http://example.org#bar')
-        self.assertEqual(ClosedNamespace('http://example.org/me#', 'foo', 'bar').foo, 'http://example.org/me#foo')
-        self.assertEqual(ClosedNamespace('http://example.org/me#', 'foo', 'bar').bar, 'http://example.org/me#bar')
-        self.assertRaises(KeyError, getattr, ClosedNamespace('http://example.org/', 'bar', 'bar'), 'foobar')
-        self.assertRaises(KeyError, getattr, ClosedNamespace('http://example.org#', 'bar', 'bar'), 'foobar')
-
-    def test_getitem(self):
-        self.assertEqual(ClosedNamespace('http://example.org/', 'foo', 'bar')['foo'], 'http://example.org#foo')
-        self.assertEqual(ClosedNamespace('http://example.org/', 'foo', 'bar')['bar'], 'http://example.org#bar')
-        self.assertEqual(ClosedNamespace('http://example.org/me#', 'foo', 'bar')['foo'], 'http://example.org/me#foo')
-        self.assertEqual(ClosedNamespace('http://example.org/me#', 'foo', 'bar')['bar'], 'http://example.org/me#bar')
-        self.assertRaises(KeyError, ClosedNamespace('http://example.org/', 'bar', 'bar').__getitem__, 'foobar')
-        self.assertRaises(KeyError, ClosedNamespace('http://example.org#', 'bar', 'bar').__getitem__, 'foobar')
+        self.assertEqual(FinalNamespace('http://example.org').foo, FinalNamespace('http://example.org#foo'))
+        self.assertEqual(FinalNamespace('http://example.org').bar, FinalNamespace('http://example.org#bar'))
+        self.assertEqual(FinalNamespace('http://example.org', sep='/').bar, FinalNamespace('http://example.org/bar'))
 
 
 ## main ##
diff --git a/test/query/__init__.py b/test/query/__init__.py
new file mode 100644
index 0000000..e69de29
--- /dev/null
+++ b/test/query/__init__.py
diff --git a/test/query/ast_test/__init__.py b/test/query/ast_test/__init__.py
new file mode 100644
index 0000000..e69de29
--- /dev/null
+++ b/test/query/ast_test/__init__.py
diff --git a/test/query/ast_test/test_fetch.py b/test/query/ast_test/test_fetch.py
new file mode 100644
index 0000000..ccb680e
--- /dev/null
+++ b/test/query/ast_test/test_fetch.py
@@ -0,0 +1,234 @@
+
+# imports
+import unittest
+
+# bsfs imports
+from bsfs.namespace import ns
+from bsfs.utils import URI
+
+# objects to test
+from bsfs.query.ast.fetch import FetchExpression
+from bsfs.query.ast.fetch import All, This
+from bsfs.query.ast.fetch import _Branch, Fetch
+from bsfs.query.ast.fetch import _Named, Node, Value
+
+
+## code ##
+
+class TestExpression(unittest.TestCase): # FetchExpression
+    def test_essentials(self):
+        class Foo(FetchExpression): pass
+        # comparison
+        self.assertEqual(FetchExpression(), FetchExpression())
+        self.assertEqual(hash(FetchExpression()), hash(FetchExpression()))
+        # comparison respects type
+        self.assertNotEqual(FetchExpression(), Foo())
+        self.assertNotEqual(hash(FetchExpression()), hash(Foo()))
+        # string conversion
+        self.assertEqual(str(FetchExpression()), 'FetchExpression()')
+        self.assertEqual(repr(FetchExpression()), 'FetchExpression()')
+        self.assertEqual(str(Foo()), 'Foo()')
+        self.assertEqual(repr(Foo()), 'Foo()')
+
+
+class TestAll(unittest.TestCase): # All
+    def test_essentials(self):
+        class Foo(All): pass
+        expr0 = This('hello')
+        expr1 = This('world')
+        # comparison
+        self.assertEqual(All(expr0), All(expr0))
+        self.assertEqual(hash(All(expr0)), hash(All(expr0)))
+        # comparison respects type
+        self.assertNotEqual(All(expr0), Foo(expr0))
+        self.assertNotEqual(hash(All(expr0)), hash(Foo(expr0)))
+        # comparison respects expressions
+        self.assertEqual(All(expr0, expr1), All(expr0, expr1))
+        self.assertEqual(hash(All(expr0, expr1)), hash(All(expr0, expr1)))
+        self.assertNotEqual(All(expr0), All(expr1))
+        self.assertNotEqual(hash(All(expr0)), hash(All(expr1)))
+        # expressions are unordered
+        self.assertEqual(All(expr0, expr1), All(expr1, expr0))
+        self.assertEqual(hash(All(expr0, expr1)), hash(All(expr1, expr0)))
+        # string conversion
+        self.assertIn(str(All(expr0, expr1)), {
+            'All({This(world), This(hello)})',
+            'All({This(hello), This(world)})'})
+        self.assertIn(repr(All(expr0, expr1)), {
+            'All({This(world), This(hello)})',
+            'All({This(hello), This(world)})'})
+
+    def test_members(self):
+        class Foo(): pass
+        expr0 = This('hello')
+        expr1 = This('world')
+        # requires at least one child expression
+        self.assertRaises(AttributeError, All)
+        # expr returns child expressions
+        self.assertEqual(All(expr0, expr1).expr, {expr0, expr1})
+        # can pass expressions as arguments
+        self.assertEqual(All(expr0, expr1).expr, {expr0, expr1})
+        # can pass a single expression as argument
+        self.assertEqual(All(expr0).expr, {expr0})
+        # can pass expressions as list-like
+        self.assertEqual(All([expr0, expr1]).expr, {expr0, expr1})
+        self.assertEqual(All((expr0, expr1)).expr, {expr0, expr1})
+        self.assertEqual(All({expr0, expr1}).expr, {expr0, expr1})
+        # can pass a single expression as list-like
+        self.assertEqual(All([expr0]).expr, {expr0})
+        # must pass a FilterExpression
+        self.assertRaises(TypeError, All, Foo())
+        self.assertRaises(TypeError, All, 1234)
+        self.assertRaises(TypeError, All, 'hello world')
+        # len returns the number of child expressions
+        self.assertEqual(len(All(expr0)), 1)
+        self.assertEqual(len(All(expr0, expr1)), 2)
+        # iter iterates over child expressions
+        self.assertSetEqual(set(All(expr0, expr1)), {expr0, expr1})
+
+
+class TestThis(unittest.TestCase): # This
+    def test_essentials(self):
+        class Foo(This): pass
+        # comparison
+        self.assertEqual(This('hello'), This('hello'))
+        self.assertEqual(hash(This('hello')), hash(This('hello')))
+        # comparison respects type
+        self.assertNotEqual(This('hello'), Foo('hello'))
+        self.assertNotEqual(hash(This('hello')), hash(Foo('hello')))
+        # comparison respects name
+        self.assertNotEqual(This('hello'), This('world'))
+        self.assertNotEqual(hash(This('hello')), hash(This('world')))
+        # string conversion
+        self.assertEqual(str(This('hello')), 'This(hello)')
+        self.assertEqual(repr(This('hello')), 'This(hello)')
+
+    def test_members(self):
+        class Foo(): pass
+        # name returns member
+        self.assertEqual(This('hello').name, 'hello')
+        self.assertEqual(This('world').name, 'world')
+        # name is converted to a string
+        self.assertEqual(This(1234).name, '1234')
+        foo = Foo()
+        self.assertEqual(This(foo).name, str(foo))
+
+
+class TestBranch(unittest.TestCase): # _Branch, Fetch
+    def test_essentials(self):
+        pred = ns.bse.tag
+        expr = FetchExpression()
+        # comparison
+        self.assertEqual(_Branch(pred), _Branch(pred))
+        self.assertEqual(hash(_Branch(pred)), hash(_Branch(pred)))
+        self.assertEqual(Fetch(pred, expr), Fetch(pred, expr))
+        self.assertEqual(hash(Fetch(pred, expr)), hash(Fetch(pred, expr)))
+        # comparison respects type
+        self.assertNotEqual(_Branch(pred), Fetch(pred, expr))
+        self.assertNotEqual(hash(_Branch(pred)), hash(Fetch(pred, expr)))
+        self.assertNotEqual(Fetch(pred, expr), _Branch(pred))
+        self.assertNotEqual(hash(Fetch(pred, expr)), hash(_Branch(pred)))
+        # comparison respects predicate
+        self.assertNotEqual(_Branch(pred), _Branch(ns.bse.filesize))
+        self.assertNotEqual(hash(_Branch(pred)), hash(_Branch(ns.bse.filesize)))
+        self.assertNotEqual(Fetch(pred, expr), Fetch(ns.bse.filesize, expr))
+        self.assertNotEqual(hash(Fetch(pred, expr)), hash(Fetch(ns.bse.filesize, expr)))
+        # comparison respects expression
+        self.assertNotEqual(Fetch(pred, expr), Fetch(pred, This('foo')))
+        self.assertNotEqual(hash(Fetch(pred, expr)), hash(Fetch(pred, This('foo'))))
+        # string conversion
+        self.assertEqual(str(_Branch(pred)), f'_Branch({pred})')
+        self.assertEqual(repr(_Branch(pred)), f'_Branch({pred})')
+        self.assertEqual(str(Fetch(pred, expr)), f'Fetch({pred}, {expr})')
+        self.assertEqual(repr(Fetch(pred, expr)), f'Fetch({pred}, {expr})')
+
+    def test_members(self):
+        class Foo(): pass
+        pred = ns.bse.tag
+        expr = FetchExpression()
+
+        # predicate returns member
+        self.assertEqual(_Branch(pred).predicate, pred)
+        self.assertEqual(Fetch(pred, expr).predicate, pred)
+        # can pass an URI
+        self.assertEqual(_Branch(ns.bse.filename).predicate, ns.bse.filename)
+        self.assertEqual(Fetch(ns.bse.filename, expr).predicate, ns.bse.filename)
+        # must pass an URI
+        self.assertRaises(TypeError, _Branch, Foo())
+        self.assertRaises(TypeError, Fetch, Foo(), expr)
+        # expression returns member
+        self.assertEqual(Fetch(pred, expr).expr, expr)
+        # expression must be a FilterExpression
+        self.assertRaises(TypeError, Fetch, ns.bse.filename, 'hello')
+        self.assertRaises(TypeError, Fetch, ns.bse.filename, 1234)
+        self.assertRaises(TypeError, Fetch, ns.bse.filename, Foo())
+
+
+class TestNamed(unittest.TestCase): # _Named, Node, Value
+    def test_essentials(self):
+        pred = ns.bse.tag
+        name = 'foobar'
+        # comparison
+        self.assertEqual(_Named(pred, name), _Named(pred, name))
+        self.assertEqual(hash(_Named(pred, name)), hash(_Named(pred, name)))
+        # comparison respects type
+        self.assertNotEqual(_Named(pred, name), Node(pred, name))
+        self.assertNotEqual(Node(pred, name), Value(pred, name))
+        self.assertNotEqual(Value(pred, name), _Named(pred, name))
+        self.assertNotEqual(hash(_Named(pred, name)), hash(Node(pred, name)))
+        self.assertNotEqual(hash(Node(pred, name)), hash(Value(pred, name)))
+        self.assertNotEqual(hash(Value(pred, name)), hash(_Named(pred, name)))
+        # comparison respects predicate
+        self.assertNotEqual(_Named(pred, name), _Named(ns.bse.filesize, name))
+        self.assertNotEqual(hash(_Named(pred, name)), hash(_Named(ns.bse.filesize, name)))
+        self.assertNotEqual(Node(pred, name), Node(ns.bse.filesize, name))
+        self.assertNotEqual(hash(Node(pred, name)), hash(Node(ns.bse.filesize, name)))
+        self.assertNotEqual(Value(pred, name), Value(ns.bse.filesize, name))
+        self.assertNotEqual(hash(Value(pred, name)), hash(Value(ns.bse.filesize, name)))
+        # comparison respects name
+        self.assertNotEqual(_Named(pred, name), _Named(pred, 'foo'))
+        self.assertNotEqual(hash(_Named(pred, name)), hash(_Named(pred, 'foo')))
+        self.assertNotEqual(Node(pred, name), Node(pred, 'foo'))
+        self.assertNotEqual(hash(Node(pred, name)), hash(Node(pred, 'foo')))
+        self.assertNotEqual(Value(pred, name), Value(pred, 'foo'))
+        self.assertNotEqual(hash(Value(pred, name)), hash(Value(pred, 'foo')))
+        # string conversion
+        self.assertEqual(str(_Named(pred, name)), f'_Named({pred}, {name})')
+        self.assertEqual(repr(_Named(pred, name)), f'_Named({pred}, {name})')
+        self.assertEqual(str(Node(pred, name)), f'Node({pred}, {name})')
+        self.assertEqual(repr(Node(pred, name)), f'Node({pred}, {name})')
+        self.assertEqual(str(Value(pred, name)), f'Value({pred}, {name})')
+        self.assertEqual(repr(Value(pred, name)), f'Value({pred}, {name})')
+
+    def test_members(self):
+        class Foo(): pass
+        pred = ns.bse.tag
+        name = 'foobar'
+        # predicate returns member
+        self.assertEqual(_Named(pred, name).predicate, pred)
+        self.assertEqual(Node(pred, name).predicate, pred)
+        self.assertEqual(Value(pred, name).predicate, pred)
+        # can pass an URI as predicate
+        self.assertEqual(_Named(ns.bse.filename, name).predicate, ns.bse.filename)
+        self.assertEqual(Node(ns.bse.filename, name).predicate, ns.bse.filename)
+        self.assertEqual(Value(ns.bse.filename, name).predicate, ns.bse.filename)
+        # must pass an URI
+        self.assertRaises(TypeError, _Named, Foo(), name)
+        self.assertRaises(TypeError, Node, Foo(), name)
+        self.assertRaises(TypeError, Value, Foo(), name)
+        # name returns member
+        self.assertEqual(_Named(pred, name).name, name)
+        self.assertEqual(Node(pred, name).name, name)
+        self.assertEqual(Value(pred, name).name, name)
+        # name is converted to a string
+        self.assertEqual(_Named(pred, 1234).name, '1234')
+        self.assertEqual(Node(pred, 1234).name, '1234')
+        self.assertEqual(Value(pred, 1234).name, '1234')
+
+
+## main ##
+
+if __name__ == '__main__':
+    unittest.main()
+
+## EOF ##
diff --git a/test/query/ast_test/test_filter_.py b/test/query/ast_test/test_filter_.py
new file mode 100644
index 0000000..d0d42ea
--- /dev/null
+++ b/test/query/ast_test/test_filter_.py
@@ -0,0 +1,614 @@
+
+# imports
+import unittest
+
+# bsfs imports
+from bsfs.namespace import ns
+from bsfs.utils import URI
+
+# objects to test
+from bsfs.query.ast.filter_ import _Expression, FilterExpression, PredicateExpression
+from bsfs.query.ast.filter_ import _Branch, Any, All
+from bsfs.query.ast.filter_ import _Agg, And, Or
+from bsfs.query.ast.filter_ import Not, Has, Distance
+from bsfs.query.ast.filter_ import _Value, Is, Equals, Substring, StartsWith, EndsWith
+from bsfs.query.ast.filter_ import _Bounded, LessThan, GreaterThan
+from bsfs.query.ast.filter_ import Predicate, OneOf
+from bsfs.query.ast.filter_ import IsIn, IsNotIn
+from bsfs.query.ast.filter_ import Includes, Excludes, Between
+
+
+## code ##
+
+class TestExpression(unittest.TestCase):
+    def test_essentials(self):
+        # comparison
+        self.assertEqual(_Expression(), _Expression())
+        self.assertEqual(FilterExpression(), FilterExpression())
+        self.assertEqual(PredicateExpression(), PredicateExpression())
+        self.assertEqual(hash(_Expression()), hash(_Expression()))
+        self.assertEqual(hash(FilterExpression()), hash(FilterExpression()))
+        self.assertEqual(hash(PredicateExpression()), hash(PredicateExpression()))
+        # comparison respects type
+        self.assertNotEqual(FilterExpression(), _Expression())
+        self.assertNotEqual(_Expression(), PredicateExpression())
+        self.assertNotEqual(PredicateExpression(), FilterExpression())
+        self.assertNotEqual(hash(FilterExpression()), hash(_Expression()))
+        self.assertNotEqual(hash(_Expression()), hash(PredicateExpression()))
+        self.assertNotEqual(hash(PredicateExpression()), hash(FilterExpression()))
+        # string conversion
+        self.assertEqual(str(_Expression()), '_Expression()')
+        self.assertEqual(str(FilterExpression()), 'FilterExpression()')
+        self.assertEqual(str(PredicateExpression()), 'PredicateExpression()')
+        self.assertEqual(repr(_Expression()), '_Expression()')
+        self.assertEqual(repr(FilterExpression()), 'FilterExpression()')
+        self.assertEqual(repr(PredicateExpression()), 'PredicateExpression()')
+
+
+class TestBranch(unittest.TestCase): # _Branch, Any, All
+    def test_essentials(self):
+        pred = PredicateExpression()
+        expr = FilterExpression()
+
+        # comparison respects type
+        self.assertNotEqual(_Branch(pred, expr), Any(pred, expr))
+        self.assertNotEqual(Any(pred, expr), All(pred, expr))
+        self.assertNotEqual(All(pred, expr), _Branch(pred, expr))
+        self.assertNotEqual(hash(_Branch(pred, expr)), hash(Any(pred, expr)))
+        self.assertNotEqual(hash(Any(pred, expr)), hash(All(pred, expr)))
+        self.assertNotEqual(hash(All(pred, expr)), hash(_Branch(pred, expr)))
+
+        for cls in (_Branch, Any, All):
+            # comparison
+            self.assertEqual(cls(pred, expr), cls(pred, expr))
+            self.assertEqual(hash(cls(pred, expr)), hash(cls(pred, expr)))
+            # comparison respects predicate
+            self.assertNotEqual(cls(ns.bse.filename, expr), cls(ns.bse.filesize, expr))
+            self.assertNotEqual(hash(cls(ns.bse.filename, expr)), hash(cls(ns.bse.filesize, expr)))
+            # comparison respects expression
+            self.assertNotEqual(cls(pred, Equals('hello')), cls(pred, Equals('world')))
+            self.assertNotEqual(hash(cls(pred, Equals('hello'))), hash(cls(pred, Equals('world'))))
+
+        # string conversion
+        self.assertEqual(str(_Branch(pred, expr)), f'_Branch({pred}, {expr})')
+        self.assertEqual(repr(_Branch(pred, expr)), f'_Branch({pred}, {expr})')
+        self.assertEqual(str(Any(pred, expr)), f'Any({pred}, {expr})')
+        self.assertEqual(repr(Any(pred, expr)), f'Any({pred}, {expr})')
+        self.assertEqual(str(All(pred, expr)), f'All({pred}, {expr})')
+        self.assertEqual(repr(All(pred, expr)), f'All({pred}, {expr})')
+
+    def test_members(self):
+        class Foo(): pass
+        pred = PredicateExpression()
+        expr = FilterExpression()
+
+        for cls in (_Branch, Any, All):
+            # predicate returns member
+            self.assertEqual(cls(PredicateExpression(), expr).predicate, PredicateExpression())
+            # can pass an URI
+            self.assertEqual(cls(ns.bse.filename, expr).predicate, Predicate(ns.bse.filename))
+            # can pass a PredicateExpression
+            self.assertEqual(cls(Predicate(ns.bse.filename), expr).predicate, Predicate(ns.bse.filename))
+            # must pass an URI or PredicateExpression
+            self.assertRaises(TypeError, cls, Foo(), expr)
+            # expression returns member
+            self.assertEqual(cls(pred, Equals('hello')).expr, Equals('hello'))
+            # expression must be a FilterExpression
+            self.assertRaises(TypeError, cls, ns.bse.filename, 'hello')
+            self.assertRaises(TypeError, cls, ns.bse.filename, 1234)
+            self.assertRaises(TypeError, cls, ns.bse.filename, Foo())
+
+
+class TestAgg(unittest.TestCase): # _Agg, And, Or
+    def test_essentials(self):
+        expr = {Equals('hello'), Equals('world')}
+
+        # comparison respects type
+        self.assertNotEqual(_Agg(expr), And(expr))
+        self.assertNotEqual(And(expr), Or(expr))
+        self.assertNotEqual(Or(expr), _Agg(expr))
+        self.assertNotEqual(hash(_Agg(expr)), hash(And(expr)))
+        self.assertNotEqual(hash(And(expr)), hash(Or(expr)))
+        self.assertNotEqual(hash(Or(expr)), hash(_Agg(expr)))
+
+        for cls in (_Agg, And, Or):
+            # comparison
+            self.assertEqual(cls(expr), cls(expr))
+            self.assertEqual(hash(cls(expr)), hash(cls(expr)))
+            # comparison respects expression
+            self.assertNotEqual(cls(expr), cls(Equals('world')))
+            self.assertNotEqual(hash(cls(expr)), hash(cls(Equals('world'))))
+            self.assertNotEqual(cls(Equals('hello')), cls(Equals('world')))
+            self.assertNotEqual(hash(cls(Equals('hello'))), hash(cls(Equals('world'))))
+
+        # string conversion
+        self.assertEqual(str(_Agg(Equals('hello'))), '_Agg({Equals(hello)})')
+        self.assertEqual(repr(_Agg(Equals('hello'))), '_Agg({Equals(hello)})')
+        self.assertEqual(str(And(Equals('hello'))), 'And({Equals(hello)})')
+        self.assertEqual(repr(And(Equals('hello'))), 'And({Equals(hello)})')
+        self.assertEqual(str(Or(Equals('hello'))), 'Or({Equals(hello)})')
+        self.assertEqual(repr(Or(Equals('hello'))), 'Or({Equals(hello)})')
+
+    def test_expression(self):
+        class Foo(): pass
+
+        for cls in (_Agg, And, Or):
+            # can pass expressions as arguments
+            self.assertSetEqual(cls(Equals('hello'), Equals('world')).expr, {Equals('hello'), Equals('world')})
+            # can pass one expressions as argument
+            self.assertSetEqual(cls(Equals('hello')).expr, {Equals('hello')})
+            # can pass expressions as iterator
+            self.assertSetEqual(cls(iter((Equals('hello'), Equals('world')))).expr, {Equals('hello'), Equals('world')})
+            # can pass expressions as generator
+            def gen():
+                yield Equals('hello')
+                yield Equals('world')
+            self.assertSetEqual(cls(gen()).expr, {Equals('hello'), Equals('world')})
+            # can pass expressions as list-like
+            self.assertSetEqual(cls((Equals('hello'), Equals('world'))).expr, {Equals('hello'), Equals('world')})
+            # can pass one expression as list-like
+            self.assertSetEqual(cls([Equals('hello')]).expr, {Equals('hello')})
+            # must pass expressions
+            self.assertRaises(TypeError, cls, Foo(), Foo())
+            self.assertRaises(TypeError, cls, [Foo(), Foo()])
+
+            # iter
+            self.assertSetEqual(set(iter(cls(Equals('hello'), Equals('world')))), {Equals('hello'), Equals('world')})
+            # contains
+            self.assertIn(Equals('world'), cls(Equals('hello'), Equals('world')))
+            self.assertNotIn(Equals('foo'), cls(Equals('hello'), Equals('world')))
+            # len
+            self.assertEqual(len(cls(Equals('hello'), Equals('world'))), 2)
+            self.assertEqual(len(cls(Equals('hello'), Equals('world'), Equals('foo'))), 3)
+
+
+
+class TestNot(unittest.TestCase):
+    def test_essentials(self):
+        expr = FilterExpression()
+        # comparison
+        self.assertEqual(Not(expr), Not(expr))
+        self.assertEqual(hash(Not(expr)), hash(Not(expr)))
+        # comparison respects type
+        self.assertNotEqual(Not(expr), FilterExpression())
+        self.assertNotEqual(hash(Not(expr)), hash(FilterExpression()))
+        # comparison respects expression
+        self.assertNotEqual(Not(Equals('hello')), Not(Equals('world')))
+        self.assertNotEqual(hash(Not(Equals('hello'))), hash(Not(Equals('world'))))
+        # string conversion
+        self.assertEqual(str(Not(Equals('hello'))), 'Not(Equals(hello))')
+        self.assertEqual(repr(Not(Equals('hello'))), 'Not(Equals(hello))')
+
+    def test_expression(self):
+        # Not requires an expression argument
+        self.assertRaises(TypeError, Not)
+        # expression must be a FilterExpression
+        self.assertRaises(TypeError, Not, 'hello')
+        self.assertRaises(TypeError, Not, 1234)
+        self.assertRaises(TypeError, Not, Predicate(ns.bse.filesize))
+        # member returns expression
+        self.assertEqual(Not(Equals('hello')).expr, Equals('hello'))
+
+
+class TestHas(unittest.TestCase):
+    def test_essentials(self):
+        pred = PredicateExpression()
+        count = FilterExpression()
+        # comparison
+        self.assertEqual(Has(pred, count), Has(pred, count))
+        self.assertEqual(hash(Has(pred, count)), hash(Has(pred, count)))
+        # comparison respects type
+        self.assertNotEqual(Has(pred, count), FilterExpression())
+        self.assertNotEqual(hash(Has(pred, count)), hash(FilterExpression()))
+        # comparison respects predicate
+        self.assertNotEqual(Has(pred, count), Has(Predicate(ns.bse.filesize), count))
+        self.assertNotEqual(hash(Has(pred, count)), hash(Has(Predicate(ns.bse.filesize), count)))
+        # comparison respects count
+        self.assertNotEqual(Has(pred, count), Has(pred, LessThan(5)))
+        self.assertNotEqual(hash(Has(pred, count)), hash(Has(pred, LessThan(5))))
+        # string conversion
+        self.assertEqual(str(Has(Predicate(ns.bse.filesize), LessThan(5))),
+            f'Has(Predicate({ns.bse.filesize}, False), LessThan(5.0, True))')
+        self.assertEqual(repr(Has(Predicate(ns.bse.filesize), LessThan(5))),
+            f'Has(Predicate({ns.bse.filesize}, False), LessThan(5.0, True))')
+
+    def test_members(self):
+        pred = PredicateExpression()
+        count = FilterExpression()
+        # member returns expression
+        # predicate must be an URI or a PredicateExpression
+        self.assertEqual(Has(ns.bse.filesize, count).predicate, Predicate(ns.bse.filesize))
+        self.assertEqual(Has(Predicate(ns.bse.filesize), count).predicate, Predicate(ns.bse.filesize))
+        self.assertRaises(TypeError, Has, 1234, FilterExpression())
+        self.assertRaises(TypeError, Has, FilterExpression(), FilterExpression())
+        # member returns count
+        # count must be None, an integer, or a FilterExpression
+        self.assertEqual(Has(pred).count, GreaterThan(1, False))
+        self.assertEqual(Has(pred, LessThan(5)).count, LessThan(5))
+        self.assertEqual(Has(pred, 5).count, Equals(5))
+        self.assertRaises(TypeError, Has, pred, 'hello')
+        self.assertRaises(TypeError, Has, pred, Predicate(ns.bse.filesize))
+
+
+
+class TestValue(unittest.TestCase):
+    def test_essentials(self):
+        # comparison respects type
+        self.assertNotEqual(_Value('hello'), Equals('hello'))
+        self.assertNotEqual(Equals('hello'), Is('hello'))
+        self.assertNotEqual(Is('hello'), Substring('hello'))
+        self.assertNotEqual(Substring('hello'), StartsWith('hello'))
+        self.assertNotEqual(StartsWith('hello'), EndsWith('hello'))
+        self.assertNotEqual(EndsWith('hello'), _Value('hello'))
+        self.assertNotEqual(hash(_Value('hello')), hash(Equals('hello')))
+        self.assertNotEqual(hash(Equals('hello')), hash(Is('hello')))
+        self.assertNotEqual(hash(Is('hello')), hash(Substring('hello')))
+        self.assertNotEqual(hash(Substring('hello')), hash(StartsWith('hello')))
+        self.assertNotEqual(hash(StartsWith('hello')), hash(EndsWith('hello')))
+        self.assertNotEqual(hash(EndsWith('hello')), hash(_Value('hello')))
+
+        for cls in (_Value, Is, Equals, Substring, StartsWith, EndsWith):
+            # comparison
+            self.assertEqual(cls('hello'), cls('hello'))
+            self.assertEqual(hash(cls('hello')), hash(cls('hello')))
+            # comparison respects value
+            self.assertNotEqual(cls('hello'), cls('world'))
+            self.assertNotEqual(hash(cls('hello')), hash(cls('world')))
+
+        # string conversion
+        self.assertEqual(str(_Value('hello')), '_Value(hello)')
+        self.assertEqual(repr(_Value('hello')), '_Value(hello)')
+        self.assertEqual(str(Is('hello')), 'Is(hello)')
+        self.assertEqual(repr(Is('hello')), 'Is(hello)')
+        self.assertEqual(str(Equals('hello')), 'Equals(hello)')
+        self.assertEqual(repr(Equals('hello')), 'Equals(hello)')
+        self.assertEqual(str(Substring('hello')), 'Substring(hello)')
+        self.assertEqual(repr(Substring('hello')), 'Substring(hello)')
+        self.assertEqual(str(StartsWith('hello')), 'StartsWith(hello)')
+        self.assertEqual(repr(StartsWith('hello')), 'StartsWith(hello)')
+        self.assertEqual(str(EndsWith('hello')), 'EndsWith(hello)')
+        self.assertEqual(repr(EndsWith('hello')), 'EndsWith(hello)')
+
+    def test_value(self):
+        class Foo(): pass
+        for cls in (_Value, Is, Equals, Substring, StartsWith, EndsWith):
+            # value can be anything
+            # value returns member
+            f = Foo()
+            self.assertEqual(cls('hello').value, 'hello')
+            self.assertEqual(cls(1234).value, 1234)
+            self.assertEqual(cls(f).value, f)
+
+
+class TestDistance(unittest.TestCase):
+    def test_essentials(self):
+        ref = (1,2,3)
+        # comparison
+        self.assertEqual(Distance(ref, 3), Distance(ref, 3))
+        self.assertEqual(hash(Distance(ref, 3)), hash(Distance(ref, 3)))
+        # comparison respects type
+        self.assertNotEqual(Distance(ref, 3), FilterExpression())
+        self.assertNotEqual(hash(Distance(ref, 3)), hash(FilterExpression()))
+        # comparison respects reference
+        self.assertNotEqual(Distance((1,2,3), 3, False), Distance((1,2), 3, False))
+        self.assertNotEqual(hash(Distance((1,2,3), 3, False)), hash(Distance((1,2), 3, False)))
+        self.assertNotEqual(Distance((1,2,3), 3, False), Distance((1,5,3), 3, False))
+        self.assertNotEqual(hash(Distance((1,2,3), 3, False)), hash(Distance((1,5,3), 3, False)))
+        # comparison respects threshold
+        self.assertNotEqual(Distance((1,2,3), 3, False), Distance((1,2,3), 3.1, False))
+        self.assertNotEqual(hash(Distance((1,2,3), 3, False)), hash(Distance((1,2,3), 3.1, False)))
+        # comparison respects strict flag
+        self.assertNotEqual(Distance((1,2,3), 3, False), Distance((1,2,3), 3, True))
+        self.assertNotEqual(hash(Distance((1,2,3), 3, False)), hash(Distance((1,2,3), 3, True)))
+        # string conversion
+        self.assertEqual(str(Distance(ref, 3, False)), 'Distance((1, 2, 3), 3.0, False)')
+        self.assertEqual(repr(Distance(ref, 3, False)), 'Distance((1, 2, 3), 3.0, False)')
+
+    def test_members(self):
+        self.assertEqual(Distance((1,2,3), 3, False).reference, (1,2,3))
+        self.assertEqual(Distance((3,2,1), 3, False).reference, (3,2,1))
+        self.assertEqual(Distance((1,2,3), 3, False).threshold, 3.0)
+        self.assertEqual(Distance((1,2,3), 53.45, False).threshold, 53.45)
+        self.assertEqual(Distance((1,2,3), 3, False).strict, False)
+        self.assertEqual(Distance((1,2,3), 3, True).strict, True)
+
+
+class TestBounded(unittest.TestCase):
+    def test_essentials(self):
+        # comparison respects type
+        self.assertNotEqual(_Bounded(1234), LessThan(1234))
+        self.assertNotEqual(LessThan(1234), GreaterThan(1234))
+        self.assertNotEqual(GreaterThan(1234), _Bounded(1234))
+        self.assertNotEqual(hash(_Bounded(1234)), hash(LessThan(1234)))
+        self.assertNotEqual(hash(LessThan(1234)), hash(GreaterThan(1234)))
+        self.assertNotEqual(hash(GreaterThan(1234)), hash(_Bounded(1234)))
+
+        for cls in (_Bounded, LessThan, GreaterThan):
+            # comparison
+            self.assertEqual(cls(1234), cls(1234))
+            self.assertEqual(hash(cls(1234)), hash(cls(1234)))
+            # comparison respects threshold
+            self.assertNotEqual(cls(1234), cls(4321))
+            self.assertNotEqual(hash(cls(1234)), hash(cls(4321)))
+            # comparison respects strict
+            self.assertNotEqual(cls(1234, True), cls(1234, False))
+            self.assertNotEqual(hash(cls(1234, True)), hash(cls(1234, False)))
+
+        # string conversion
+        self.assertEqual(str(_Bounded(1234, False)), '_Bounded(1234.0, False)')
+        self.assertEqual(repr(_Bounded(1234, False)), '_Bounded(1234.0, False)')
+        self.assertEqual(str(LessThan(1234, False)), 'LessThan(1234.0, False)')
+        self.assertEqual(repr(LessThan(1234, False)), 'LessThan(1234.0, False)')
+        self.assertEqual(str(GreaterThan(1234, False)), 'GreaterThan(1234.0, False)')
+        self.assertEqual(repr(GreaterThan(1234, False)), 'GreaterThan(1234.0, False)')
+
+    def test_members(self):
+        class Foo(): pass
+        for cls in (_Bounded, LessThan, GreaterThan):
+            # threshold becomes float
+            self.assertEqual(cls(1.234).threshold, 1.234)
+            self.assertEqual(cls(1234).threshold, 1234.0)
+            self.assertEqual(cls('1234').threshold, 1234)
+            self.assertRaises(TypeError, cls, Foo())
+            # strict becomes bool
+            self.assertEqual(cls(1234, True).strict, True)
+            self.assertEqual(cls(1234, False).strict, False)
+            self.assertEqual(cls(1234, Foo()).strict, True)
+
+
+class TestPredicate(unittest.TestCase):
+    def test_essentials(self):
+        # comparison
+        self.assertEqual(Predicate(ns.bse.filesize), Predicate(ns.bse.filesize))
+        self.assertEqual(hash(Predicate(ns.bse.filesize)), hash(Predicate(ns.bse.filesize)))
+        # comparison respects type
+        self.assertNotEqual(Predicate(ns.bse.filesize), PredicateExpression())
+        self.assertNotEqual(hash(Predicate(ns.bse.filesize)), hash(PredicateExpression()))
+        # comparison respects predicate
+        self.assertNotEqual(Predicate(ns.bse.filesize), Predicate(ns.bse.filename))
+        self.assertNotEqual(hash(Predicate(ns.bse.filesize)), hash(Predicate(ns.bse.filename)))
+        # comparison respects reverse
+        self.assertNotEqual(Predicate(ns.bse.filesize, True), Predicate(ns.bse.filesize, False))
+        self.assertNotEqual(hash(Predicate(ns.bse.filesize, True)), hash(Predicate(ns.bse.filesize, False)))
+        # string conversion
+        self.assertEqual(str(Predicate(ns.bse.filesize)), f'Predicate({ns.bse.filesize}, False)')
+        self.assertEqual(str(Predicate(ns.bse.filesize, True)),
+            f'Predicate({ns.bse.filesize}, True)')
+        self.assertEqual(repr(Predicate(ns.bse.filesize)), f'Predicate({ns.bse.filesize}, False)')
+        self.assertEqual(repr(Predicate(ns.bse.filesize, True)),
+            f'Predicate({ns.bse.filesize}, True)')
+
+    def test_members(self):
+        # member returns predicate
+        # predicate must be an URI
+        self.assertEqual(Predicate(ns.bse.filesize).predicate, ns.bse.filesize)
+        self.assertEqual(Predicate(URI('hello_world')).predicate, URI('hello_world'))
+        self.assertRaises(TypeError, Predicate, 1234)
+        self.assertRaises(TypeError, Predicate, FilterExpression())
+        self.assertRaises(TypeError, Predicate, FilterExpression())
+        # reverse becomes a boolean
+        self.assertEqual(Predicate(ns.bse.filesize, True).reverse, True)
+        self.assertEqual(Predicate(ns.bse.filesize, False).reverse, False)
+        self.assertEqual(Predicate(ns.bse.filesize, 'abc').reverse, True)
+
+
+class TestOneOf(unittest.TestCase):
+    def test_essentials(self):
+        expr = {Predicate(ns.bse.filename), Predicate(ns.bse.filesize)}
+        # comparison
+        self.assertEqual(OneOf(expr), OneOf(expr))
+        self.assertEqual(hash(OneOf(expr)), hash(OneOf(expr)))
+        # comparison respects type
+        self.assertNotEqual(OneOf(expr), PredicateExpression())
+        self.assertNotEqual(hash(OneOf(expr)), hash(PredicateExpression()))
+        # comparison respects expression
+        self.assertNotEqual(OneOf(expr), OneOf(Predicate(ns.bse.filename)))
+        self.assertNotEqual(hash(OneOf(expr)), hash(OneOf(Predicate(ns.bse.filename))))
+        # string conversion
+        self.assertEqual(str(OneOf(Predicate(ns.bse.filesize))),
+            f'OneOf({{Predicate({ns.bse.filesize}, False)}})')
+        self.assertEqual(repr(OneOf(Predicate(ns.bse.filesize))),
+            f'OneOf({{Predicate({ns.bse.filesize}, False)}})')
+
+    def test_expression(self):
+        class Foo(): pass
+        # can pass expressions as arguments
+        self.assertSetEqual(OneOf(Predicate(ns.bse.filesize), Predicate(ns.bse.filename)).expr,
+            {Predicate(ns.bse.filesize), Predicate(ns.bse.filename)})
+        # can pass one expressions as argument
+        self.assertSetEqual(OneOf(Predicate(ns.bse.filesize)).expr,
+            {Predicate(ns.bse.filesize)})
+        # can pass expressions as iterator
+        self.assertSetEqual(OneOf(iter((Predicate(ns.bse.filesize), Predicate(ns.bse.filename)))).expr,
+            {Predicate(ns.bse.filesize), Predicate(ns.bse.filename)})
+        # can pass expressions as generator
+        def gen():
+            yield Predicate(ns.bse.filesize)
+            yield Predicate(ns.bse.filename)
+        self.assertSetEqual(OneOf(gen()).expr,
+            {Predicate(ns.bse.filesize), Predicate(ns.bse.filename)})
+        # can pass expressions as list-like
+        self.assertSetEqual(OneOf((Predicate(ns.bse.filesize), Predicate(ns.bse.filename))).expr,
+            {Predicate(ns.bse.filesize), Predicate(ns.bse.filename)})
+        # can pass one expression as list-like
+        self.assertSetEqual(OneOf([Predicate(ns.bse.filesize)]).expr,
+            {Predicate(ns.bse.filesize)})
+        # must pass expressions
+        self.assertRaises(TypeError, OneOf, Foo(), Foo())
+        self.assertRaises(TypeError, OneOf, [Foo(), Foo()])
+        # must pass at least one expression
+        self.assertRaises(AttributeError, OneOf)
+
+        # iter
+        self.assertSetEqual(set(iter(OneOf(Predicate(ns.bse.filesize), Predicate(ns.bse.filename)))),
+            {Predicate(ns.bse.filesize), Predicate(ns.bse.filename)})
+        # contains
+        self.assertIn(Predicate(ns.bse.filesize),
+            OneOf(Predicate(ns.bse.filesize), Predicate(ns.bse.filename)))
+        self.assertNotIn(Predicate(ns.bse.tag),
+            OneOf(Predicate(ns.bse.filesize), Predicate(ns.bse.filename)))
+        # len
+        self.assertEqual(len(OneOf(Predicate(ns.bse.filesize), Predicate(ns.bse.filename))), 2)
+        self.assertEqual(len(OneOf(Predicate(ns.bse.filesize), Predicate(ns.bse.filename), Predicate(ns.bse.tag))), 3)
+
+
+    def test_IsIn(self):
+        # cannot pass zero arguments
+        self.assertRaises(AttributeError, IsIn)
+        # can pass expressions as arguments
+        self.assertEqual(IsIn('http://example.com/entity#1234', 'http://example.com/entity#4321'),
+            Or(Is('http://example.com/entity#1234'), Is('http://example.com/entity#4321')))
+        # can pass one expression as argument
+        self.assertEqual(IsIn('http://example.com/entity#1234'),
+            Is('http://example.com/entity#1234'))
+        # can pass expressions as iterator
+        self.assertEqual(IsIn(iter(('http://example.com/entity#1234', 'http://example.com/entity#4321'))),
+            Or(Is('http://example.com/entity#1234'), Is('http://example.com/entity#4321')))
+        # can pass expressions as generator
+        def gen():
+            yield 'http://example.com/entity#1234'
+            yield 'http://example.com/entity#4321'
+        self.assertEqual(IsIn(gen()),
+            Or(Is('http://example.com/entity#1234'), Is('http://example.com/entity#4321')))
+        # can pass expressions as list-like
+        self.assertEqual(IsIn(['http://example.com/entity#1234', 'http://example.com/entity#4321']),
+            Or(Is('http://example.com/entity#1234'), Is('http://example.com/entity#4321')))
+        # can pass one expression as list-like
+        self.assertEqual(IsIn(['http://example.com/entity#1234']),
+            Is('http://example.com/entity#1234'))
+
+
+    def test_IsNotIn(self):
+        # cannot pass zero arguments
+        self.assertRaises(AttributeError, IsNotIn)
+        # can pass expressions as arguments
+        self.assertEqual(IsNotIn('http://example.com/entity#1234', 'http://example.com/entity#4321'),
+            Not(Or(Is('http://example.com/entity#1234'), Is('http://example.com/entity#4321'))))
+        # can pass one expression as argument
+        self.assertEqual(IsNotIn('http://example.com/entity#1234'),
+            Not(Is('http://example.com/entity#1234')))
+        # can pass expressions as iterator
+        self.assertEqual(IsNotIn(iter(('http://example.com/entity#1234', 'http://example.com/entity#4321'))),
+            Not(Or(Is('http://example.com/entity#1234'), Is('http://example.com/entity#4321'))))
+        # can pass expressions as generator
+        def gen():
+            yield 'http://example.com/entity#1234'
+            yield 'http://example.com/entity#4321'
+        self.assertEqual(IsNotIn(gen()),
+            Not(Or(Is('http://example.com/entity#1234'), Is('http://example.com/entity#4321'))))
+        # can pass expressions as list-like
+        self.assertEqual(IsNotIn(['http://example.com/entity#1234', 'http://example.com/entity#4321']),
+            Not(Or(Is('http://example.com/entity#1234'), Is('http://example.com/entity#4321'))))
+        # can pass one expression as list-like
+        self.assertEqual(IsNotIn(['http://example.com/entity#1234']),
+            Not(Is('http://example.com/entity#1234')))
+
+
+    def test_Includes(self):
+        # cannot pass zero arguments
+        self.assertRaises(AttributeError, Includes)
+        # can pass expressions as arguments
+        self.assertEqual(Includes('hello', 'world'),
+            Or(Equals('hello'), Equals('world')))
+        self.assertEqual(Includes('hello', 'world', approx=True),
+            Or(Substring('hello'), Substring('world')))
+        # can pass one expression as argument
+        self.assertEqual(Includes('hello'),
+            Equals('hello'))
+        self.assertEqual(Includes('hello', approx=True),
+            Substring('hello'))
+        # can pass expressions as iterator
+        self.assertEqual(Includes(iter(('hello', 'world'))),
+            Or(Equals('hello'), Equals('world')))
+        self.assertEqual(Includes(iter(('hello', 'world')), approx=True),
+            Or(Substring('hello'), Substring('world')))
+        # can pass expressions as generator
+        def gen():
+            yield 'hello'
+            yield 'world'
+        self.assertEqual(Includes(gen()),
+            Or(Equals('hello'), Equals('world')))
+        self.assertEqual(Includes(gen(), approx=True),
+            Or(Substring('hello'), Substring('world')))
+        # can pass expressions as list-like
+        self.assertEqual(Includes(['hello', 'world']),
+            Or(Equals('hello'), Equals('world')))
+        self.assertEqual(Includes(['hello', 'world'], approx=True),
+            Or(Substring('hello'), Substring('world')))
+        # can pass one expression as list-like
+        self.assertEqual(Includes(['hello']),
+            Equals('hello'))
+        self.assertEqual(Includes(['hello'], approx=True),
+            Substring('hello'))
+
+
+    def test_Excludes(self):
+        # cannot pass zero arguments
+        self.assertRaises(AttributeError, Excludes)
+        # can pass expressions as arguments
+        self.assertEqual(Excludes('hello', 'world'),
+            Not(Or(Equals('hello'), Equals('world'))))
+        self.assertEqual(Excludes('hello', 'world', approx=True),
+            Not(Or(Substring('hello'), Substring('world'))))
+        # can pass one expression as argument
+        self.assertEqual(Excludes('hello'),
+            Not(Equals('hello')))
+        self.assertEqual(Excludes('hello', approx=True),
+            Not(Substring('hello')))
+        # can pass expressions as iterator
+        self.assertEqual(Excludes(iter(('hello', 'world'))),
+            Not(Or(Equals('hello'), Equals('world'))))
+        self.assertEqual(Excludes(iter(('hello', 'world')), approx=True),
+            Not(Or(Substring('hello'), Substring('world'))))
+        # can pass expressions as generator
+        def gen():
+            yield 'hello'
+            yield 'world'
+        self.assertEqual(Excludes(gen()),
+            Not(Or(Equals('hello'), Equals('world'))))
+        self.assertEqual(Excludes(gen(), approx=True),
+            Not(Or(Substring('hello'), Substring('world'))))
+        # can pass expressions as list-like
+        self.assertEqual(Excludes(['hello', 'world']),
+            Not(Or(Equals('hello'), Equals('world'))))
+        self.assertEqual(Excludes(['hello', 'world'], approx=True),
+            Not(Or(Substring('hello'), Substring('world'))))
+        # can pass one expression as list-like
+        self.assertEqual(Excludes(['hello']),
+            Not(Equals('hello')))
+        self.assertEqual(Excludes(['hello'], approx=True),
+            Not(Substring('hello')))
+
+
+    def test_Between(self):
+        # must specify at least one bound
+        self.assertRaises(ValueError, Between, float('inf'), float('inf'))
+        # lower bound must be less than the upper bound
+        self.assertRaises(ValueError, Between, 321, 123)
+        # can set a lower bound only
+        self.assertEqual(Between(123),
+            GreaterThan(123, strict=True))
+        self.assertEqual(Between(123, lo_strict=False),
+            GreaterThan(123, strict=False))
+        # can set an upper bound only
+        self.assertEqual(Between(hi=123),
+            LessThan(123, strict=True))
+        self.assertEqual(Between(hi=123, hi_strict=False),
+            LessThan(123, strict=False))
+        # can set both bounds
+        self.assertEqual(Between(123, 321),
+            And(GreaterThan(123, strict=True), LessThan(321, strict=True)))
+        self.assertEqual(Between(123, 321, False, False),
+            And(GreaterThan(123, strict=False), LessThan(321, strict=False)))
+        # can set identical bounds
+        self.assertRaises(ValueError, Between, 123, 123)
+        self.assertEqual(Between(123, 123, False, False),
+            Equals(123))
+
+
+## main ##
+
+if __name__ == '__main__':
+    unittest.main()
+
+## EOF ##
diff --git a/test/query/test_matcher.py b/test/query/test_matcher.py
new file mode 100644
index 0000000..6b975b2
--- /dev/null
+++ b/test/query/test_matcher.py
@@ -0,0 +1,1177 @@
+
+# imports
+import operator
+import unittest
+
+# bsfs imports
+from bsfs.namespace import ns
+from bsfs.query import ast
+from bsfs.utils import errors
+
+# objects to test
+from bsfs.query.matcher import Any, Filter, Partial, Rest, _set_matcher
+
+
+## code ##
+
+class TestAny(unittest.TestCase):
+    def test_essentials(self):
+        # comparison
+        a = Any()
+        b = Any()
+        self.assertNotEqual(Any(), Any())
+        self.assertNotEqual(hash(Any()), hash(Any()))
+        self.assertNotEqual(a, Any())
+        self.assertNotEqual(hash(a), hash(Any()))
+        self.assertNotEqual(a, b)
+        self.assertNotEqual(hash(a), hash(b))
+        # comparison within sets
+        self.assertEqual(len({Any(), Any(), Any(), Any()}), 4)
+        self.assertEqual(len({Any() for _ in range(1000)}), 1000)
+        # string representation
+        self.assertEqual(str(Any()), 'Any()')
+        self.assertEqual(repr(Any()), 'Any()')
+
+
+class TestRest(unittest.TestCase):
+    def test_essentials(self):
+        expr = ast.filter.Equals('hello')
+        # comparison
+        self.assertEqual(Rest(expr), Rest(expr))
+        self.assertEqual(hash(Rest(expr)), hash(Rest(expr)))
+        # comparison respects type
+        class Foo(): pass
+        self.assertNotEqual(Rest(expr), 1234)
+        self.assertNotEqual(hash(Rest(expr)), hash(1234))
+        self.assertNotEqual(Rest(expr), Foo())
+        self.assertNotEqual(hash(Rest(expr)), hash(Foo()))
+        # comparison respects expr
+        self.assertNotEqual(Rest(expr), Rest(ast.filter.Equals('world')))
+        self.assertNotEqual(hash(Rest(expr)), hash(Rest(ast.filter.Equals('world'))))
+        # default constructor -> Any -> Not equal
+        self.assertNotEqual(Rest(), Rest())
+        self.assertNotEqual(hash(Rest()), hash(Rest()))
+        # string representation
+        self.assertEqual(str(Rest()), 'Rest(Any())')
+        self.assertEqual(str(Rest(expr)), 'Rest(Equals(hello))')
+        self.assertEqual(repr(Rest()), 'Rest(Any())')
+        self.assertEqual(repr(Rest(expr)), 'Rest(Equals(hello))')
+
+
+
+class TestPartial(unittest.TestCase):
+    def test_match(self):
+        p0 = Partial(ast.filter.LessThan)
+        p1 = Partial(ast.filter.LessThan, threshold=3)
+        p2 = Partial(ast.filter.LessThan, strict=False)
+        p3 = Partial(ast.filter.LessThan, threshold=3, strict=False)
+        # match respects name
+        self.assertTrue(p0.match('foo', None))
+        self.assertTrue(p1.match('foo', None))
+        self.assertTrue(p2.match('foo', None))
+        self.assertTrue(p3.match('foo', None))
+        # match respects correct value
+        self.assertTrue(p0.match('threshold', 3))
+        self.assertTrue(p1.match('threshold', 3))
+        self.assertTrue(p2.match('threshold', 3))
+        self.assertTrue(p3.match('threshold', 3))
+        self.assertTrue(p0.match('strict', False))
+        self.assertTrue(p1.match('strict', False))
+        self.assertTrue(p2.match('strict', False))
+        self.assertTrue(p3.match('strict', False))
+        # match respects incorrect value
+        self.assertTrue(p0.match('threshold', 5))
+        self.assertFalse(p1.match('threshold', 5))
+        self.assertTrue(p2.match('threshold', 5))
+        self.assertFalse(p3.match('threshold', 5))
+        self.assertTrue(p0.match('strict', True))
+        self.assertTrue(p1.match('strict', True))
+        self.assertFalse(p2.match('strict', True))
+        self.assertFalse(p3.match('strict', True))
+
+    def test_members(self):
+        # node returns expression
+        self.assertEqual(Partial(ast.filter.Equals).node, ast.filter.Equals)
+        self.assertEqual(Partial(ast.filter.LessThan).node, ast.filter.LessThan)
+        # kwargs returns arguments
+        self.assertDictEqual(Partial(ast.filter.Equals, value='hello').kwargs,
+            {'value': 'hello'})
+        self.assertDictEqual(Partial(ast.filter.LessThan, threshold=3, strict=False).kwargs,
+            {'threshold': 3, 'strict': False})
+        # Partial does not check about kwargs
+        self.assertDictEqual(Partial(ast.filter.LessThan, value='hello').kwargs,
+            {'value': 'hello'})
+        self.assertDictEqual(Partial(ast.filter.Equals, threshold=3, strict=False).kwargs,
+            {'threshold': 3, 'strict': False})
+
+    def test_essentials(self):
+        # comparison respects type
+        class Foo(): pass
+        self.assertNotEqual(Partial(ast.filter.Equals), 1234)
+        self.assertNotEqual(hash(Partial(ast.filter.Equals)), hash(1234))
+        self.assertNotEqual(Partial(ast.filter.Equals), Foo())
+        self.assertNotEqual(hash(Partial(ast.filter.Equals)), hash(Foo()))
+        self.assertNotEqual(Partial(ast.filter.Equals), ast.filter.Equals)
+        self.assertNotEqual(hash(Partial(ast.filter.Equals)), hash(ast.filter.Equals))
+        self.assertNotEqual(Partial(ast.filter.Equals), ast.filter.Equals('hello'))
+        self.assertNotEqual(hash(Partial(ast.filter.Equals)), hash(ast.filter.Equals('hello')))
+        # comparison respects node
+        self.assertEqual(Partial(ast.filter.Equals), Partial(ast.filter.Equals))
+        self.assertEqual(hash(Partial(ast.filter.Equals)), hash(Partial(ast.filter.Equals)))
+        self.assertEqual(Partial(ast.filter.LessThan), Partial(ast.filter.LessThan))
+        self.assertEqual(hash(Partial(ast.filter.LessThan)), hash(Partial(ast.filter.LessThan)))
+        self.assertNotEqual(Partial(ast.filter.Equals), Partial(ast.filter.LessThan))
+        self.assertNotEqual(hash(Partial(ast.filter.Equals)), hash(Partial(ast.filter.LessThan)))
+        # comparison respects kwargs
+        self.assertEqual(
+            Partial(ast.filter.Equals, value='hello'),
+            Partial(ast.filter.Equals, value='hello'))
+        self.assertEqual(
+            hash(Partial(ast.filter.Equals, value='hello')),
+            hash(Partial(ast.filter.Equals, value='hello')))
+        self.assertEqual(
+            Partial(ast.filter.LessThan, threshold=3, strict=False),
+            Partial(ast.filter.LessThan, threshold=3, strict=False))
+        self.assertEqual(
+            hash(Partial(ast.filter.LessThan, threshold=3, strict=False)),
+            hash(Partial(ast.filter.LessThan, threshold=3, strict=False)))
+        self.assertNotEqual(
+            Partial(ast.filter.Equals, value='hello'),
+            Partial(ast.filter.Equals))
+        self.assertNotEqual(
+            hash(Partial(ast.filter.Equals, value='hello')),
+            hash(Partial(ast.filter.Equals)))
+        self.assertNotEqual(
+            Partial(ast.filter.Equals, value='hello'),
+            Partial(ast.filter.Equals, value='world'))
+        self.assertNotEqual(
+            hash(Partial(ast.filter.Equals, value='hello')),
+            hash(Partial(ast.filter.Equals, value='world')))
+        self.assertNotEqual(
+            Partial(ast.filter.LessThan, threshold=3, strict=False),
+            Partial(ast.filter.LessThan))
+        self.assertNotEqual(
+            hash(Partial(ast.filter.LessThan, threshold=3, strict=False)),
+            hash(Partial(ast.filter.LessThan)))
+        self.assertNotEqual(
+            Partial(ast.filter.LessThan, threshold=3, strict=False),
+            Partial(ast.filter.LessThan, threshold=5))
+        self.assertNotEqual(
+            hash(Partial(ast.filter.LessThan, threshold=3, strict=False)),
+            hash(Partial(ast.filter.LessThan, threshold=5)))
+        self.assertNotEqual(
+            Partial(ast.filter.LessThan, threshold=3, strict=False),
+            Partial(ast.filter.LessThan, strict=False))
+        self.assertNotEqual(
+            hash(Partial(ast.filter.LessThan, threshold=3, strict=False)),
+            hash(Partial(ast.filter.LessThan, strict=False)))
+        self.assertNotEqual(
+            Partial(ast.filter.LessThan, threshold=3, strict=False),
+            Partial(ast.filter.LessThan, threshold=3, strict=True))
+        self.assertNotEqual(
+            hash(Partial(ast.filter.LessThan, threshold=3, strict=False)),
+            hash(Partial(ast.filter.LessThan, threshold=3, strict=True)))
+        self.assertNotEqual(
+            Partial(ast.filter.LessThan, threshold=3, strict=False),
+            Partial(ast.filter.LessThan, threshold=5, strict=False))
+        self.assertNotEqual(
+            hash(Partial(ast.filter.LessThan, threshold=3, strict=False)),
+            hash(Partial(ast.filter.LessThan, threshold=5, strict=False)))
+        # string representation
+        self.assertEqual(str(Partial(ast.filter.Equals)), 'Partial(Equals, {})')
+        self.assertEqual(repr(Partial(ast.filter.Equals)), 'Partial(Equals, {})')
+        self.assertEqual(str(Partial(ast.filter.LessThan)), 'Partial(LessThan, {})')
+        self.assertEqual(repr(Partial(ast.filter.LessThan)), 'Partial(LessThan, {})')
+        self.assertEqual(str(Partial(ast.filter.Equals, value='hello')), "Partial(Equals, {'value': 'hello'})")
+        self.assertEqual(repr(Partial(ast.filter.Equals, value='hello')), "Partial(Equals, {'value': 'hello'})")
+        self.assertEqual(str(Partial(ast.filter.LessThan, threshold=3)), "Partial(LessThan, {'threshold': 3})")
+        self.assertEqual(repr(Partial(ast.filter.LessThan, threshold=3)), "Partial(LessThan, {'threshold': 3})")
+        self.assertEqual(str(Partial(ast.filter.LessThan, strict=False)), "Partial(LessThan, {'strict': False})")
+        self.assertEqual(repr(Partial(ast.filter.LessThan, strict=False)), "Partial(LessThan, {'strict': False})")
+        self.assertEqual(str(Partial(ast.filter.LessThan, threshold=3, strict=False)), "Partial(LessThan, {'threshold': 3, 'strict': False})")
+        self.assertEqual(repr(Partial(ast.filter.LessThan, threshold=3, strict=False)), "Partial(LessThan, {'threshold': 3, 'strict': False})")
+
+
+class TestSetMatcher(unittest.TestCase):
+    def test_set_matcher(self):
+        # setup
+        A = ast.filter.Equals('A')
+        B = ast.filter.Equals('B')
+        C = ast.filter.Equals('C')
+        D = ast.filter.Equals('D')
+        matcher = Filter()
+
+        # identical sets match
+        self.assertTrue(_set_matcher(
+            ast.filter.And(A, B, C),
+            ast.filter.And(A, B, C),
+            matcher._parse_filter_expression,
+            ))
+
+        # order is irrelevant
+        self.assertTrue(_set_matcher(
+            ast.filter.And(A, B, C),
+            ast.filter.And(B, C, A),
+            matcher._parse_filter_expression,
+            ))
+
+        # all reference items must be present
+        self.assertFalse(_set_matcher(
+            ast.filter.And(A, B),
+            ast.filter.And(A, B, C),
+            matcher._parse_filter_expression,
+            ))
+
+        # all reference items must have a match
+        self.assertFalse(_set_matcher(
+            ast.filter.And(A, B, C),
+            ast.filter.And(D, B, C),
+            matcher._parse_filter_expression,
+            ))
+        self.assertFalse(_set_matcher(
+            ast.filter.And(A, B, C),
+            ast.filter.And(A, D, C),
+            matcher._parse_filter_expression,
+            ))
+        self.assertFalse(_set_matcher(
+            ast.filter.And(A, B, C),
+            ast.filter.And(A, B, D),
+            matcher._parse_filter_expression,
+            ))
+
+        # Any matches every item
+        self.assertTrue(_set_matcher(
+            ast.filter.And(A, B, C),
+            ast.filter.And(Any(), B, C),
+            matcher._parse_filter_expression,
+            ))
+        self.assertTrue(_set_matcher(
+            ast.filter.And(A, B, C),
+            ast.filter.And(A, Any(), C),
+            matcher._parse_filter_expression,
+            ))
+        self.assertTrue(_set_matcher(
+            ast.filter.And(A, B, C),
+            ast.filter.And(A, B, Any()),
+            matcher._parse_filter_expression,
+            ))
+        self.assertTrue(_set_matcher(
+            ast.filter.And(A, B, D),
+            ast.filter.And(A, B, Any()),
+            matcher._parse_filter_expression,
+            ))
+
+        # there can be multiple Any's
+        self.assertTrue(_set_matcher(
+            ast.filter.And(A, B, C),
+            ast.filter.And(A, Any(), Any()),
+            matcher._parse_filter_expression,
+            ))
+        self.assertTrue(_set_matcher(
+            ast.filter.And(A, B, C),
+            ast.filter.And(Any(), B, Any()),
+            matcher._parse_filter_expression,
+            ))
+        self.assertTrue(_set_matcher(
+            ast.filter.And(A, B, C),
+            ast.filter.And(Any(), Any(), C),
+            matcher._parse_filter_expression,
+            ))
+
+        # Any covers exactly one element
+        self.assertTrue(_set_matcher(
+            ast.filter.And(A, B, C),
+            ast.filter.And(A, B, Any()),
+            matcher._parse_filter_expression,
+            ))
+        self.assertTrue(_set_matcher(
+            ast.filter.And(A, B, D),
+            ast.filter.And(A, B, Any()),
+            matcher._parse_filter_expression,
+            ))
+        self.assertFalse(_set_matcher(
+            ast.filter.And(A, B),
+            ast.filter.And(A, B, Any()),
+            matcher._parse_filter_expression,
+            ))
+        self.assertFalse(_set_matcher(
+            ast.filter.And(A, B, C, D),
+            ast.filter.And(A, B, Any()),
+            matcher._parse_filter_expression,
+            ))
+
+        # each Any covers exactly one element
+        self.assertTrue(_set_matcher(
+            ast.filter.And(A, B, C),
+            ast.filter.And(Any(), Any(), Any()),
+            matcher._parse_filter_expression,
+            ))
+        self.assertFalse(_set_matcher(
+            ast.filter.And(A, B, C),
+            ast.filter.And(Any(), Any()),
+            matcher._parse_filter_expression,
+            ))
+        self.assertFalse(_set_matcher(
+            ast.filter.And(A, B),
+            ast.filter.And(Any(), Any(), Any()),
+            matcher._parse_filter_expression,
+            ))
+
+        # Rest captures remainder
+        self.assertTrue(_set_matcher(
+            ast.filter.And(A, B, C),
+            ast.filter.And(A, B, Rest()),
+            matcher._parse_filter_expression,
+            ))
+        self.assertTrue(_set_matcher(
+            ast.filter.And(A, B, C, D),
+            ast.filter.And(A, B, Rest()),
+            matcher._parse_filter_expression,
+            ))
+        # remainder matches the empty set
+        self.assertTrue(_set_matcher(
+            ast.filter.And(A, B),
+            ast.filter.And(A, B, Rest()),
+            matcher._parse_filter_expression,
+            ))
+        # Rest does not absolve other refernce items from having a match
+        self.assertFalse(_set_matcher(
+            ast.filter.And(A, C, D),
+            ast.filter.And(A, B, Rest()),
+            matcher._parse_filter_expression,
+            ))
+        # Rest can be combined with Any ...
+        self.assertTrue(_set_matcher(
+            ast.filter.And(A, C, D),
+            ast.filter.And(A, Any(), Rest()),
+            matcher._parse_filter_expression,
+            ))
+        self.assertTrue(_set_matcher(
+            ast.filter.And(A, C, D),
+            ast.filter.And(A, Any(), Rest()),
+            matcher._parse_filter_expression,
+            ))
+        # ... explicit items still need to match
+        self.assertFalse(_set_matcher(
+            ast.filter.And(A, C, D),
+            ast.filter.And(B, Any(), Rest()),
+            matcher._parse_filter_expression,
+            ))
+        # ... Any still determines minimum element count
+        self.assertTrue(_set_matcher(
+            ast.filter.And(A, B),
+            ast.filter.And(A, Any(), Rest()),
+            matcher._parse_filter_expression,
+            ))
+        self.assertFalse(_set_matcher(
+            ast.filter.And(A, B),
+            ast.filter.And(A, Any(), Any(), Rest()),
+            matcher._parse_filter_expression,
+            ))
+        # Rest cannot be repeated ...
+        self.assertRaises(errors.BackendError, _set_matcher,
+            ast.filter.And(A, B, C),
+            ast.filter.And(A, Rest(), Rest(ast.filter.Equals('hello'))),
+            matcher._parse_filter_expression,
+            )
+        # ... unless they are identical
+        self.assertRaises(errors.BackendError, _set_matcher,
+            ast.filter.And(A, B, C),
+            ast.filter.And(A, Rest(), Rest()), # Any instances are different!
+            matcher._parse_filter_expression,
+            )
+        # ... unless they are identical
+        self.assertTrue(_set_matcher(
+            ast.filter.And(A, B, C),
+            ast.filter.And(A, B, Rest(C), Rest(C)),
+            matcher._parse_filter_expression,
+            ))
+        # Rest can mandate a specific expression
+        self.assertTrue(_set_matcher(
+            ast.filter.And(A, B, C),
+            ast.filter.And(A, B, Rest(C)),
+            matcher._parse_filter_expression,
+            ))
+        self.assertFalse(_set_matcher(
+            ast.filter.And(A, B, C),
+            ast.filter.And(A, B, Rest(D)),
+            matcher._parse_filter_expression,
+            ))
+        # Rest can mandate a partial expression
+        self.assertTrue(_set_matcher(
+            ast.filter.And(A, B, C),
+            ast.filter.And(A, B, Rest(Partial(ast.filter.Equals))),
+            matcher._parse_filter_expression,
+            ))
+        self.assertTrue(_set_matcher(
+            ast.filter.And(A, B, C),
+            ast.filter.And(A, Rest(Partial(ast.filter.Equals))),
+            matcher._parse_filter_expression,
+            ))
+        self.assertFalse(_set_matcher(
+            ast.filter.And(A, B, C),
+            ast.filter.And(A, B, Rest(Partial(ast.filter.Substring))),
+            matcher._parse_filter_expression,
+            ))
+        self.assertFalse(_set_matcher(
+            ast.filter.And(A, B, C),
+            ast.filter.And(A, B, Rest(Partial(ast.filter.Equals, value='D'))),
+            matcher._parse_filter_expression,
+            ))
+        # Rest can be the only expression
+        self.assertTrue(_set_matcher(
+            ast.filter.And(A, B, C),
+            ast.filter.And(Rest(Partial(ast.filter.Equals))),
+            matcher._parse_filter_expression,
+            ))
+        # Rest's expression defaults to Any
+        self.assertTrue(_set_matcher(
+            ast.filter.And(A, B, C),
+            ast.filter.And(Rest()),
+            matcher._parse_filter_expression,
+            ))
+
+
+class TestFilter(unittest.TestCase):
+    def setUp(self):
+        self.match = Filter()
+
+    def test_call(self):
+        # query must be a filter expression
+        self.assertRaises(errors.BackendError, self.match, 1234, Any())
+        self.assertRaises(errors.BackendError, self.match, ast.filter.Predicate(ns.bse.filename), Any())
+        # reference must be a filter expression
+        self.assertRaises(errors.BackendError, self.match, ast.filter.Equals('hello'), 1234)
+        self.assertRaises(errors.BackendError, self.match, ast.filter.Equals('hello'), ast.filter.Predicate(ns.bse.filename))
+        # reference can be Any or Partial
+        self.assertTrue(self.match(
+            ast.filter.Equals('hello'),
+            Any(),
+            ))
+        self.assertTrue(self.match(
+            ast.filter.Equals('hello'),
+            Partial(ast.filter.Equals),
+            ))
+        # call parses expression
+        self.assertTrue(self.match(
+            # query
+            ast.filter.And(
+                ast.filter.Any(ns.bse.tag,
+                    ast.filter.All(ns.bse.label,
+                        ast.filter.Or(
+                            ast.filter.Equals('hello'),
+                            ast.filter.Equals('world'),
+                            ast.filter.StartsWith('foo'),
+                            ast.filter.EndsWith('bar'),
+                            )
+                        )
+                    ),
+                ast.filter.Any(ns.bse.iso,
+                    ast.filter.And(
+                        ast.filter.GreaterThan(100, strict=True),
+                        ast.filter.LessThan(200, strict=False),
+                        )
+                    ),
+                ast.filter.Any(ast.filter.OneOf(ns.bse.featureA, ns.bse.featureB),
+                    ast.filter.Distance([1,2,3], 1)
+                    ),
+                ),
+            # reference
+            ast.filter.And(
+                ast.filter.Any(Any(),
+                    ast.filter.All(Partial(ast.filter.Predicate, reverse=False),
+                        ast.filter.Or(
+                            Partial(ast.filter.StartsWith),
+                            ast.filter.EndsWith('bar'),
+                            Rest(Partial(ast.filter.Equals)),
+                            )
+                        )
+                    ),
+                ast.filter.Any(ns.bse.iso,
+                    ast.filter.And(
+                        Partial(ast.filter.GreaterThan, strict=True),
+                        Any(),
+                        Rest(),
+                        )
+                    ),
+                ast.filter.Any(ast.filter.OneOf(Rest()),
+                    Partial(ast.filter.Distance)
+                    ),
+                ),
+            ))
+        self.assertFalse(self.match(
+            # query
+            ast.filter.Any(ns.bse.tag,
+                ast.filter.And(
+                    ast.filter.Any(ns.bse.label, ast.filter.Equals('hello')),
+                    ast.filter.Any(ns.bse.collection, ast.filter.Is('http://example.com/col#123')),
+                    ast.filter.Not(ast.filter.Has(ns.bse.label)),
+                    )
+                ),
+            # reference
+            ast.filter.Any(ns.bse.tag,
+                ast.filter.And(
+                    Any(),
+                    ast.filter.Any(Partial(ast.filter.Predicate, reverse=True), # reverse mismatch
+                        Partial(ast.filter.Is)),
+                    ast.filter.Not(ast.filter.Has(Any(), Any())),
+                    )
+                )
+            ))
+
+    def test_parse_filter_expression(self):
+        # Any matches every filter expression
+        self.assertTrue(self.match._parse_filter_expression(
+            ast.filter.Not(ast.filter.FilterExpression()), Any()))
+        self.assertTrue(self.match._parse_filter_expression(
+            ast.filter.Has(ns.bse.filename), Any()))
+        self.assertTrue(self.match._parse_filter_expression(
+            ast.filter.Distance([1,2,3], 1.0), Any()))
+        self.assertTrue(self.match._parse_filter_expression(
+            ast.filter.Any(ns.bse.filename, ast.filter.Equals('hello')), Any()))
+        self.assertTrue(self.match._parse_filter_expression(
+            ast.filter.All(ns.bse.filename, ast.filter.Equals('hello')), Any()))
+        self.assertTrue(self.match._parse_filter_expression(
+            ast.filter.And(ast.filter.Equals('hello')), Any()))
+        self.assertTrue(self.match._parse_filter_expression(
+            ast.filter.Or(ast.filter.Equals('hello')), Any()))
+        self.assertTrue(self.match._parse_filter_expression(
+            ast.filter.Equals('hello'), Any()))
+        self.assertTrue(self.match._parse_filter_expression(
+            ast.filter.Substring('hello'), Any()))
+        self.assertTrue(self.match._parse_filter_expression(
+            ast.filter.StartsWith('hello'), Any()))
+        self.assertTrue(self.match._parse_filter_expression(
+            ast.filter.EndsWith('hello'), Any()))
+        self.assertTrue(self.match._parse_filter_expression(
+            ast.filter.Is('hello'), Any()))
+        self.assertTrue(self.match._parse_filter_expression(
+            ast.filter.LessThan(3), Any()))
+        self.assertTrue(self.match._parse_filter_expression(
+            ast.filter.GreaterThan(3), Any()))
+        # Any matches invalid filter expressions
+        self.assertTrue(self.match._parse_filter_expression(
+            ast.filter.FilterExpression(), Any()))
+        # node must be an appropriate filter expression
+        self.assertRaises(errors.BackendError, self.match._parse_filter_expression,
+            ast.filter.FilterExpression(), ast.filter.FilterExpression())
+        self.assertRaises(errors.BackendError, self.match._parse_filter_expression,
+            1234, ast.filter.FilterExpression())
+
+    def test_parse_predicate_expression(self):
+        # Any matches every predicate expression
+        self.assertTrue(self.match._parse_predicate_expression(
+            ast.filter.Predicate(ns.bse.filename), Any()))
+        self.assertTrue(self.match._parse_predicate_expression(
+            ast.filter.OneOf(ns.bse.filename), Any()))
+        # Any matches invalid predicate expression
+        self.assertTrue(self.match._parse_predicate_expression(
+            ast.filter.FilterExpression(), Any()))
+        # node must be an appropriate predicate expression
+        self.assertRaises(errors.BackendError, self.match._parse_predicate_expression,
+            ast.filter.PredicateExpression(), ast.filter.PredicateExpression())
+        self.assertRaises(errors.BackendError, self.match._parse_predicate_expression,
+            1234, ast.filter.PredicateExpression())
+
+    def test_predicate(self):
+        # identical expressions match
+        self.assertTrue(self.match._predicate(
+            ast.filter.Predicate(ns.bse.filename, reverse=False),
+            ast.filter.Predicate(ns.bse.filename, reverse=False),
+            ))
+        # _predicate respects type
+        self.assertFalse(self.match._predicate(
+            ast.filter.Predicate(ns.bse.filename, reverse=False),
+            ast.filter.FilterExpression(),
+            ))
+        # _predicate respects predicate
+        self.assertFalse(self.match._predicate(
+            ast.filter.Predicate(ns.bse.filename, reverse=False),
+            ast.filter.Predicate(ns.bse.filesize, reverse=False),
+            ))
+        # _predicate respects reverse
+        self.assertFalse(self.match._predicate(
+            ast.filter.Predicate(ns.bse.filename, reverse=False),
+            ast.filter.Predicate(ns.bse.filename, reverse=True),
+            ))
+        # Partial requires ast.filter.Predicate
+        self.assertFalse(self.match._predicate(
+            ast.filter.Predicate(ns.bse.filename, reverse=False),
+            Partial(ast.filter.Equals),
+            ))
+        # predicate and reverse can be specified
+        self.assertTrue(self.match._predicate(
+            ast.filter.Predicate(ns.bse.filename, reverse=False),
+            Partial(ast.filter.Predicate, predicate=ns.bse.filename, reverse=False),
+            ))
+        self.assertFalse(self.match._predicate(
+            ast.filter.Predicate(ns.bse.filename, reverse=False),
+            Partial(ast.filter.Predicate, predicate=ns.bse.filesize, reverse=False),
+            ))
+        self.assertFalse(self.match._predicate(
+            ast.filter.Predicate(ns.bse.filename, reverse=False),
+            Partial(ast.filter.Predicate, predicate=ns.bse.filename, reverse=True),
+            ))
+        # predicate can remain unspecified
+        self.assertTrue(self.match._predicate(
+            ast.filter.Predicate(ns.bse.filename, reverse=False),
+            Partial(ast.filter.Predicate, reverse=False),
+            ))
+        self.assertTrue(self.match._predicate(
+            ast.filter.Predicate(ns.bse.filesize, reverse=False),
+            Partial(ast.filter.Predicate, reverse=False),
+            ))
+        self.assertFalse(self.match._predicate(
+            ast.filter.Predicate(ns.bse.filesize, reverse=False),
+            Partial(ast.filter.Predicate, reverse=True),
+            ))
+        # reverse can remain unspecified
+        self.assertTrue(self.match._predicate(
+            ast.filter.Predicate(ns.bse.filename, reverse=False),
+            Partial(ast.filter.Predicate, predicate=ns.bse.filename),
+            ))
+        self.assertTrue(self.match._predicate(
+            ast.filter.Predicate(ns.bse.filename, reverse=True),
+            Partial(ast.filter.Predicate, predicate=ns.bse.filename),
+            ))
+        self.assertFalse(self.match._predicate(
+            ast.filter.Predicate(ns.bse.filename, reverse=False),
+            Partial(ast.filter.Predicate, predicate=ns.bse.filesize),
+            ))
+
+    def test_one_of(self):
+        A = ast.filter.Predicate(ns.bse.filename)
+        B = ast.filter.Predicate(ns.bse.filesize)
+        C = ast.filter.Predicate(ns.bse.filename, reverse=True)
+        # identical expressions match
+        self.assertTrue(self.match._one_of(
+            ast.filter.OneOf(A, B),
+            ast.filter.OneOf(A, B),
+            ))
+        # _one_of respects type
+        self.assertFalse(self.match._one_of(
+            ast.filter.OneOf(A, B),
+            ast.filter.Predicate(ns.bse.filesize, reverse=True),
+            ))
+        # _one_of respects child expressions
+        self.assertFalse(self.match._one_of(
+            ast.filter.OneOf(A, B),
+            ast.filter.OneOf(A, C),
+            ))
+        self.assertFalse(self.match._one_of(
+            ast.filter.OneOf(A, B),
+            ast.filter.OneOf(A),
+            ))
+        self.assertFalse(self.match._one_of(
+            ast.filter.OneOf(A, B),
+            ast.filter.OneOf(A, B, C),
+            ))
+        self.assertTrue(self.match._one_of(
+            ast.filter.OneOf(A, B),
+            ast.filter.OneOf(B, A),
+            ))
+        self.assertTrue(self.match._one_of(
+            ast.filter.OneOf(A, B),
+            ast.filter.OneOf(A, Any()),
+            ))
+        self.assertTrue(self.match._one_of(
+            ast.filter.OneOf(A, B),
+            ast.filter.OneOf(B, Rest()),
+            ))
+
+    def test_branch(self):
+        # identical expressions match
+        self.assertTrue(self.match._branch(
+            ast.filter.Any(ns.bse.filename, ast.filter.Equals('hello')),
+            ast.filter.Any(ns.bse.filename, ast.filter.Equals('hello')),
+            ))
+        self.assertTrue(self.match._branch(
+            ast.filter.All(ns.bse.filename, ast.filter.Equals('hello')),
+            ast.filter.All(ns.bse.filename, ast.filter.Equals('hello')),
+            ))
+        # _agg respects type
+        self.assertFalse(self.match._branch(
+            ast.filter.Any(ns.bse.filename, ast.filter.Equals('hello')),
+            ast.filter.All(ns.bse.filename, ast.filter.Equals('hello')),
+            ))
+        self.assertFalse(self.match._branch(
+            ast.filter.All(ns.bse.filename, ast.filter.Equals('hello')),
+            ast.filter.Any(ns.bse.filename, ast.filter.Equals('hello')),
+            ))
+        self.assertFalse(self.match._branch(
+            ast.filter.Any(ns.bse.filename, ast.filter.Equals('hello')),
+            ast.filter.Equals('hello'),
+            ))
+        self.assertFalse(self.match._branch(
+            ast.filter.All(ns.bse.filename, ast.filter.Equals('hello')),
+            ast.filter.Equals('hello'),
+            ))
+        # _agg respects predicate expression
+        self.assertTrue(self.match._branch(
+            ast.filter.Any(ns.bse.filename, ast.filter.Equals('hello')),
+            ast.filter.Any(ast.filter.Predicate(ns.bse.filename), ast.filter.Equals('hello')),
+            ))
+        self.assertTrue(self.match._branch(
+            ast.filter.All(ns.bse.filename, ast.filter.Equals('hello')),
+            ast.filter.All(ast.filter.Predicate(ns.bse.filename), ast.filter.Equals('hello')),
+            ))
+        self.assertFalse(self.match._branch(
+            ast.filter.Any(ns.bse.filename, ast.filter.Equals('hello')),
+            ast.filter.Any(ns.bse.filesize, ast.filter.Equals('hello')),
+            ))
+        self.assertFalse(self.match._branch(
+            ast.filter.All(ns.bse.filename, ast.filter.Equals('hello')),
+            ast.filter.All(ns.bse.filesize, ast.filter.Equals('hello')),
+            ))
+        self.assertFalse(self.match._branch(
+            ast.filter.Any(ns.bse.filename, ast.filter.Equals('hello')),
+            ast.filter.Any(ast.filter.OneOf(ns.bse.filename), ast.filter.Equals('hello')),
+            ))
+        self.assertFalse(self.match._branch(
+            ast.filter.All(ns.bse.filename, ast.filter.Equals('hello')),
+            ast.filter.All(ast.filter.OneOf(ns.bse.filename), ast.filter.Equals('hello')),
+            ))
+        self.assertFalse(self.match._branch(
+            ast.filter.Any(ns.bse.filename, ast.filter.Equals('hello')),
+            ast.filter.Any(ast.filter.Predicate(ns.bse.filename, reverse=True), ast.filter.Equals('hello')),
+            ))
+        self.assertFalse(self.match._branch(
+            ast.filter.All(ns.bse.filename, ast.filter.Equals('hello')),
+            ast.filter.All(ast.filter.Predicate(ns.bse.filename, reverse=True), ast.filter.Equals('hello')),
+            ))
+        self.assertTrue(self.match._branch(
+            ast.filter.Any(ns.bse.filename, ast.filter.Equals('hello')),
+            ast.filter.Any(Any(), ast.filter.Equals('hello')),
+            ))
+        self.assertTrue(self.match._branch(
+            ast.filter.All(ns.bse.filename, ast.filter.Equals('hello')),
+            ast.filter.All(Any(), ast.filter.Equals('hello')),
+            ))
+        self.assertTrue(self.match._branch(
+            ast.filter.Any(ns.bse.filename, ast.filter.Equals('hello')),
+            ast.filter.Any(Partial(ast.filter.Predicate), ast.filter.Equals('hello')),
+            ))
+        self.assertTrue(self.match._branch(
+            ast.filter.All(ns.bse.filename, ast.filter.Equals('hello')),
+            ast.filter.All(Partial(ast.filter.Predicate), ast.filter.Equals('hello')),
+            ))
+        # _agg respects filter expression
+        self.assertFalse(self.match._branch(
+            ast.filter.Any(ns.bse.filename, ast.filter.Equals('hello')),
+            ast.filter.Any(ns.bse.filename, ast.filter.Substring('hello')),
+            ))
+        self.assertFalse(self.match._branch(
+            ast.filter.All(ns.bse.filename, ast.filter.Equals('hello')),
+            ast.filter.All(ns.bse.filename, ast.filter.Substring('hello')),
+            ))
+        self.assertFalse(self.match._branch(
+            ast.filter.Any(ns.bse.filename, ast.filter.Equals('hello')),
+            ast.filter.Any(ns.bse.filename, ast.filter.Any(Any(), Any())),
+            ))
+        self.assertFalse(self.match._branch(
+            ast.filter.All(ns.bse.filename, ast.filter.Equals('hello')),
+            ast.filter.All(ns.bse.filename, ast.filter.All(Any(), Any())),
+            ))
+        self.assertTrue(self.match._branch(
+            ast.filter.Any(ns.bse.filename, ast.filter.Equals('hello')),
+            ast.filter.Any(ns.bse.filename, Any()),
+            ))
+        self.assertTrue(self.match._branch(
+            ast.filter.All(ns.bse.filename, ast.filter.Equals('hello')),
+            ast.filter.All(ns.bse.filename, Any()),
+            ))
+        self.assertTrue(self.match._branch(
+            ast.filter.Any(ns.bse.filename, ast.filter.Equals('hello')),
+            ast.filter.Any(ns.bse.filename, Partial(ast.filter.Equals)),
+            ))
+        self.assertTrue(self.match._branch(
+            ast.filter.All(ns.bse.filename, ast.filter.Equals('hello')),
+            ast.filter.All(ns.bse.filename, Partial(ast.filter.Equals)),
+            ))
+        self.assertFalse(self.match._branch(
+            ast.filter.Any(ns.bse.filename, ast.filter.Equals('hello')),
+            ast.filter.Any(ns.bse.filename, Partial(ast.filter.Equals, value='world')),
+            ))
+        self.assertFalse(self.match._branch(
+            ast.filter.All(ns.bse.filename, ast.filter.Equals('hello')),
+            ast.filter.All(ns.bse.filename, Partial(ast.filter.Equals, value='world')),
+            ))
+
+    def test_agg(self):
+        A = ast.filter.Equals('hello')
+        B = ast.filter.Equals('world')
+        C = ast.filter.Equals('foobar')
+        # identical expressions match
+        self.assertTrue(self.match._agg(
+            ast.filter.And(A, B),
+            ast.filter.And(A, B),
+            ))
+        self.assertTrue(self.match._agg(
+            ast.filter.Or(A, B),
+            ast.filter.Or(A, B),
+            ))
+        # _agg respects type
+        self.assertFalse(self.match._agg(
+            ast.filter.And(A, B),
+            ast.filter.Or(A, B),
+            ))
+        self.assertFalse(self.match._agg(
+            ast.filter.Or(A, B),
+            ast.filter.And(A, B),
+            ))
+        self.assertFalse(self.match._agg(
+            ast.filter.And(A, B),
+            ast.filter.Equals('hello'),
+            ))
+        self.assertFalse(self.match._agg(
+            ast.filter.Or(A, B),
+            ast.filter.Equals('hello'),
+            ))
+        # _agg respects child expressions
+        self.assertFalse(self.match._agg(
+            ast.filter.And(A, B),
+            ast.filter.And(A, ast.filter.Equals('bar')),
+            ))
+        self.assertFalse(self.match._agg(
+            ast.filter.Or(A, B),
+            ast.filter.Or(A, ast.filter.Equals('bar')),
+            ))
+        self.assertFalse(self.match._agg(
+            ast.filter.And(A, B),
+            ast.filter.And(A),
+            ))
+        self.assertFalse(self.match._agg(
+            ast.filter.Or(A, B),
+            ast.filter.Or(A),
+            ))
+        self.assertFalse(self.match._agg(
+            ast.filter.And(A, B),
+            ast.filter.And(A, B, C),
+            ))
+        self.assertFalse(self.match._agg(
+            ast.filter.Or(A, B),
+            ast.filter.Or(A, B, C),
+            ))
+        self.assertTrue(self.match._agg(
+            ast.filter.And(A, B),
+            ast.filter.And(B, A),
+            ))
+        self.assertTrue(self.match._agg(
+            ast.filter.Or(A, B),
+            ast.filter.Or(B, A),
+            ))
+        self.assertTrue(self.match._agg(
+            ast.filter.And(A, B),
+            ast.filter.And(A, Any()),
+            ))
+        self.assertTrue(self.match._agg(
+            ast.filter.Or(A, B),
+            ast.filter.Or(A, Any()),
+            ))
+        self.assertTrue(self.match._agg(
+            ast.filter.And(A, B),
+            ast.filter.And(B, Rest()),
+            ))
+        self.assertTrue(self.match._agg(
+            ast.filter.Or(A, B),
+            ast.filter.Or(B, Rest()),
+            ))
+
+    def test_not(self):
+        # identical expressions match
+        self.assertTrue(self.match._not(
+            ast.filter.Not(ast.filter.Equals('hello')),
+            ast.filter.Not(ast.filter.Equals('hello')),
+            ))
+        # _not respects type
+        self.assertFalse(self.match._not(
+            ast.filter.Not(ast.filter.Equals('hello')),
+            ast.filter.Equals('hello'),
+            ))
+        # _not respects child expression
+        self.assertFalse(self.match._not(
+            ast.filter.Not(ast.filter.Equals('hello')),
+            ast.filter.Not(ast.filter.Equals('world')),
+            ))
+        self.assertFalse(self.match._not(
+            ast.filter.Not(ast.filter.Equals('hello')),
+            ast.filter.Not(ast.filter.Substring('hello')),
+            ))
+        self.assertTrue(self.match._not(
+            ast.filter.Not(ast.filter.Equals('hello')),
+            ast.filter.Not(Any()),
+            ))
+
+    def test_has(self):
+        # identical expressions match
+        self.assertTrue(self.match._has(
+            ast.filter.Has(ns.bse.filesize),
+            ast.filter.Has(ns.bse.filesize),
+            ))
+        self.assertTrue(self.match._has(
+            ast.filter.Has(ns.bse.filesize, ast.filter.LessThan(3)),
+            ast.filter.Has(ns.bse.filesize, ast.filter.LessThan(3)),
+            ))
+        # _has respects type
+        self.assertFalse(self.match._has(
+            ast.filter.Has(ns.bse.filesize),
+            ast.filter.Equals('hello'),
+            ))
+        self.assertFalse(self.match._has(
+            ast.filter.Has(ns.bse.filesize),
+            ast.filter.Equals('hello'),
+            ))
+        # _has respects predicate
+        self.assertFalse(self.match._has(
+            ast.filter.Has(ns.bse.filesize, ast.filter.LessThan(3)),
+            ast.filter.Has(ns.bse.iso, ast.filter.LessThan(3)),
+            ))
+        self.assertTrue(self.match._has(
+            ast.filter.Has(ns.bse.filesize, ast.filter.LessThan(3)),
+            ast.filter.Has(Any(), ast.filter.LessThan(3)),
+            ))
+        self.assertTrue(self.match._has(
+            ast.filter.Has(ns.bse.filesize, ast.filter.LessThan(3)),
+            ast.filter.Has(Partial(ast.filter.Predicate), ast.filter.LessThan(3)),
+            ))
+        # _has respects count
+        self.assertFalse(self.match._has(
+            ast.filter.Has(ns.bse.filesize, ast.filter.LessThan(3)),
+            ast.filter.Has(ns.bse.filesize, ast.filter.GreaterThan(3)),
+            ))
+        self.assertFalse(self.match._has(
+            ast.filter.Has(ns.bse.filesize, ast.filter.LessThan(3)),
+            ast.filter.Has(ns.bse.filesize, ast.filter.LessThan(5)),
+            ))
+        self.assertTrue(self.match._has(
+            ast.filter.Has(ns.bse.filesize, ast.filter.LessThan(3)),
+            ast.filter.Has(ns.bse.filesize, Any()),
+            ))
+        self.assertTrue(self.match._has(
+            ast.filter.Has(ns.bse.filesize, ast.filter.LessThan(3)),
+            ast.filter.Has(ns.bse.filesize, Partial(ast.filter.LessThan)),
+            ))
+
+    def test_distance(self):
+        # identical expressions match
+        self.assertTrue(self.match._distance(
+            ast.filter.Distance([1,2,3], 5, True),
+            ast.filter.Distance([1,2,3], 5, True),
+            ))
+        # _distance respects type
+        self.assertFalse(self.match._distance(
+            ast.filter.Distance([1,2,3], 5, True),
+            ast.filter.Equals('hello'),
+            ))
+        self.assertFalse(self.match._distance(
+            ast.filter.Distance([1,2,3], 5, True),
+            Partial(ast.filter.Equals),
+            ))
+        # _distance respects reference value
+        self.assertFalse(self.match._distance(
+            ast.filter.Distance([1,2,3], 5, True),
+            ast.filter.Distance([3,2,1], 5, True),
+            ))
+        self.assertTrue(self.match._distance(
+            ast.filter.Distance([1,2,3], 5, True),
+            Partial(ast.filter.Distance, threshold=5, strict=True),
+            ))
+        self.assertTrue(self.match._distance(
+            ast.filter.Distance([1,2,3], 5, True),
+            Partial(ast.filter.Distance, reference=[1,2,3], threshold=5, strict=True),
+            ))
+        self.assertFalse(self.match._distance(
+            ast.filter.Distance([1,2,3], 5, True),
+            Partial(ast.filter.Distance, reference=[3,2,1], threshold=5, strict=True),
+            ))
+        # _distance respects threshold
+        self.assertFalse(self.match._distance(
+            ast.filter.Distance([1,2,3], 5, True),
+            ast.filter.Distance([1,2,3], 8, True),
+            ))
+        self.assertTrue(self.match._distance(
+            ast.filter.Distance([1,2,3], 5, True),
+            Partial(ast.filter.Distance, reference=[1,2,3], strict=True),
+            ))
+        self.assertTrue(self.match._distance(
+            ast.filter.Distance([1,2,3], 5, True),
+            Partial(ast.filter.Distance, reference=[1,2,3], threshold=5, strict=True),
+            ))
+        self.assertFalse(self.match._distance(
+            ast.filter.Distance([1,2,3], 5, True),
+            Partial(ast.filter.Distance, reference=[1,2,3], threshold=8, strict=True),
+            ))
+        # _distance respects strict
+        self.assertFalse(self.match._distance(
+            ast.filter.Distance([1,2,3], 5, True),
+            ast.filter.Distance([1,2,3], 5, False),
+            ))
+        self.assertTrue(self.match._distance(
+            ast.filter.Distance([1,2,3], 5, True),
+            Partial(ast.filter.Distance, reference=[1,2,3], threshold=5),
+            ))
+        self.assertTrue(self.match._distance(
+            ast.filter.Distance([1,2,3], 5, True),
+            Partial(ast.filter.Distance, reference=[1,2,3], threshold=5, strict=True),
+            ))
+        self.assertFalse(self.match._distance(
+            ast.filter.Distance([1,2,3], 5, True),
+            Partial(ast.filter.Distance, reference=[1,2,3], threshold=5, strict=False),
+            ))
+
+    def test_value(self):
+        # identical expressions match
+        self.assertTrue(self.match._value(ast.filter.Equals('hello'), ast.filter.Equals('hello')))
+        self.assertTrue(self.match._value(ast.filter.Substring('hello'), ast.filter.Substring('hello')))
+        self.assertTrue(self.match._value(ast.filter.StartsWith('hello'), ast.filter.StartsWith('hello')))
+        self.assertTrue(self.match._value(ast.filter.EndsWith('hello'), ast.filter.EndsWith('hello')))
+        self.assertTrue(self.match._value(ast.filter.Is('hello'), ast.filter.Is('hello')))
+        # _value respects type
+        self.assertFalse(self.match._value(ast.filter.Equals('hello'), ast.filter.Is('hello')))
+        self.assertFalse(self.match._value(ast.filter.Substring('hello'), ast.filter.Is('hello')))
+        self.assertFalse(self.match._value(ast.filter.StartsWith('hello'), ast.filter.Is('hello')))
+        self.assertFalse(self.match._value(ast.filter.EndsWith('hello'), ast.filter.Is('hello')))
+        self.assertFalse(self.match._value(ast.filter.Is('hello'), ast.filter.Equals('hello')))
+        # _value respects value
+        self.assertFalse(self.match._value(ast.filter.Equals('hello'), ast.filter.Equals('world')))
+        self.assertFalse(self.match._value(ast.filter.Substring('hello'), ast.filter.Substring('world')))
+        self.assertFalse(self.match._value(ast.filter.StartsWith('hello'), ast.filter.StartsWith('world')))
+        self.assertFalse(self.match._value(ast.filter.EndsWith('hello'), ast.filter.EndsWith('world')))
+        self.assertFalse(self.match._value(ast.filter.Is('hello'), ast.filter.Is('world')))
+        # Partial requires correct type
+        self.assertFalse(self.match._value(ast.filter.Equals('hello'), Partial(ast.filter.Is)))
+        self.assertFalse(self.match._value(ast.filter.Substring('hello'), Partial(ast.filter.Is)))
+        self.assertFalse(self.match._value(ast.filter.StartsWith('hello'), Partial(ast.filter.Is)))
+        self.assertFalse(self.match._value(ast.filter.EndsWith('hello'), Partial(ast.filter.Is)))
+        self.assertFalse(self.match._value(ast.filter.Is('hello'), Partial(ast.filter.Equals)))
+        # value can be specified
+        self.assertTrue(self.match._value(ast.filter.Equals('hello'), Partial(ast.filter.Equals, value='hello')))
+        self.assertFalse(self.match._value(ast.filter.Equals('hello'), Partial(ast.filter.Equals, value='world')))
+        self.assertTrue(self.match._value(ast.filter.Substring('hello'), Partial(ast.filter.Substring, value='hello')))
+        self.assertFalse(self.match._value(ast.filter.Substring('hello'), Partial(ast.filter.Substring, value='world')))
+        self.assertTrue(self.match._value(ast.filter.StartsWith('hello'), Partial(ast.filter.StartsWith, value='hello')))
+        self.assertFalse(self.match._value(ast.filter.StartsWith('hello'), Partial(ast.filter.StartsWith, value='world')))
+        self.assertTrue(self.match._value(ast.filter.EndsWith('hello'), Partial(ast.filter.EndsWith, value='hello')))
+        self.assertFalse(self.match._value(ast.filter.EndsWith('hello'), Partial(ast.filter.EndsWith, value='world')))
+        self.assertTrue(self.match._value(ast.filter.Is('hello'), Partial(ast.filter.Is, value='hello')))
+        self.assertFalse(self.match._value(ast.filter.Is('hello'), Partial(ast.filter.Is, value='world')))
+        # value can remain unspecified
+        self.assertTrue(self.match._value(ast.filter.Equals('hello'), Partial(ast.filter.Equals)))
+        self.assertTrue(self.match._value(ast.filter.Substring('hello'), Partial(ast.filter.Substring)))
+        self.assertTrue(self.match._value(ast.filter.StartsWith('hello'), Partial(ast.filter.StartsWith)))
+        self.assertTrue(self.match._value(ast.filter.EndsWith('hello'), Partial(ast.filter.EndsWith)))
+        self.assertTrue(self.match._value(ast.filter.Is('hello'), Partial(ast.filter.Is)))
+
+    def test_bounded(self):
+        # identical expressions match
+        self.assertTrue(self.match._bounded(
+            ast.filter.LessThan(threshold=3, strict=False),
+            ast.filter.LessThan(threshold=3, strict=False),
+            ))
+        self.assertTrue(self.match._bounded(
+            ast.filter.GreaterThan(threshold=3, strict=False),
+            ast.filter.GreaterThan(threshold=3, strict=False),
+            ))
+        # _bounded respects type
+        self.assertFalse(self.match._bounded(
+            ast.filter.LessThan(threshold=3, strict=False),
+            ast.filter.GreaterThan(threshold=3, strict=False),
+            ))
+        self.assertFalse(self.match._bounded(
+            ast.filter.GreaterThan(threshold=3, strict=False),
+            ast.filter.LessThan(threshold=3, strict=False),
+            ))
+        # _bounded respects threshold
+        self.assertFalse(self.match._bounded(
+            ast.filter.LessThan(threshold=3, strict=False),
+            ast.filter.LessThan(threshold=4, strict=False),
+            ))
+        self.assertFalse(self.match._bounded(
+            ast.filter.GreaterThan(threshold=3, strict=False),
+            ast.filter.GreaterThan(threshold=4, strict=False),
+            ))
+        # _bounded respects strict
+        self.assertFalse(self.match._bounded(
+            ast.filter.LessThan(threshold=3, strict=False),
+            ast.filter.LessThan(threshold=3, strict=True),
+            ))
+        self.assertFalse(self.match._bounded(
+            ast.filter.GreaterThan(threshold=3, strict=False),
+            ast.filter.GreaterThan(threshold=3, strict=True),
+            ))
+        # Partial requires correct type
+        self.assertFalse(self.match._bounded(
+            ast.filter.LessThan(threshold=3, strict=False),
+            Partial(ast.filter.GreaterThan),
+            ))
+        self.assertFalse(self.match._bounded(
+            ast.filter.GreaterThan(threshold=3, strict=False),
+            Partial(ast.filter.LessThan),
+            ))
+        # threshold and strict can be specified
+        self.assertTrue(self.match._bounded(
+            ast.filter.LessThan(threshold=3, strict=False),
+            Partial(ast.filter.LessThan, threshold=3, strict=False),
+            ))
+        self.assertTrue(self.match._bounded(
+            ast.filter.GreaterThan(threshold=3, strict=False),
+            Partial(ast.filter.GreaterThan, threshold=3, strict=False),
+            ))
+        self.assertFalse(self.match._bounded(
+            ast.filter.LessThan(threshold=3, strict=False),
+            Partial(ast.filter.LessThan, threshold=4, strict=False),
+            ))
+        self.assertFalse(self.match._bounded(
+            ast.filter.GreaterThan(threshold=3, strict=False),
+            Partial(ast.filter.GreaterThan, threshold=4, strict=False),
+            ))
+        self.assertFalse(self.match._bounded(
+            ast.filter.LessThan(threshold=3, strict=False),
+            Partial(ast.filter.LessThan, threshold=3, strict=True),
+            ))
+        self.assertFalse(self.match._bounded(
+            ast.filter.GreaterThan(threshold=3, strict=False),
+            Partial(ast.filter.GreaterThan, threshold=3, strict=True),
+            ))
+        # threshold can remain unspecified
+        self.assertTrue(self.match._bounded(
+            ast.filter.LessThan(threshold=3, strict=False),
+            Partial(ast.filter.LessThan, strict=False),
+            ))
+        self.assertTrue(self.match._bounded(
+            ast.filter.GreaterThan(threshold=3, strict=False),
+            Partial(ast.filter.GreaterThan, strict=False),
+            ))
+        self.assertFalse(self.match._bounded(
+            ast.filter.LessThan(threshold=3, strict=False),
+            Partial(ast.filter.LessThan, strict=True),
+            ))
+        self.assertFalse(self.match._bounded(
+            ast.filter.GreaterThan(threshold=3, strict=False),
+            Partial(ast.filter.GreaterThan, strict=True),
+            ))
+        # strict can remain unspecified
+        self.assertTrue(self.match._bounded(
+            ast.filter.LessThan(threshold=3, strict=False),
+            Partial(ast.filter.LessThan, threshold=3),
+            ))
+        self.assertTrue(self.match._bounded(
+            ast.filter.GreaterThan(threshold=3, strict=False),
+            Partial(ast.filter.GreaterThan, threshold=3),
+            ))
+        self.assertFalse(self.match._bounded(
+            ast.filter.LessThan(threshold=3, strict=False),
+            Partial(ast.filter.LessThan, threshold=4),
+            ))
+        self.assertFalse(self.match._bounded(
+            ast.filter.GreaterThan(threshold=3, strict=False),
+            Partial(ast.filter.GreaterThan, threshold=4),
+            ))
+
+
+## main ##
+
+if __name__ == '__main__':
+    unittest.main()
+
+## EOF ##
diff --git a/test/query/test_validator.py b/test/query/test_validator.py
new file mode 100644
index 0000000..418463e
--- /dev/null
+++ b/test/query/test_validator.py
@@ -0,0 +1,505 @@
+
+# imports
+import unittest
+
+# bsfs imports
+from bsfs import schema as _schema
+from bsfs.namespace import ns
+from bsfs.query import ast
+from bsfs.utils import errors
+
+# objects to test
+from bsfs.query.validator import Filter, Fetch
+
+
+## code ##
+
+ns.bse = ns.bsfs.Entity()
+
+class TestFilter(unittest.TestCase):
+    def setUp(self):
+        self.schema = _schema.from_string('''
+            prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
+            prefix xsd: <http://www.w3.org/2001/XMLSchema#>
+
+            prefix bsfs: <https://schema.bsfs.io/core/>
+            prefix bse: <https://schema.bsfs.io/core/Entity#>
+            prefix bsl: <https://schema.bsfs.io/core/Literal/>
+
+            bsfs:Entity rdfs:subClassOf bsfs:Node .
+            bsfs:URI rdfs:subClassOf bsfs:Literal .
+
+            bsfs:Tag rdfs:subClassOf bsfs:Node .
+            xsd:string rdfs:subClassOf bsfs:Literal .
+            bsl:Number rdfs:subClassOf bsfs:Literal .
+            bsl:Array rdfs:subClassOf bsfs:Literal .
+            <https://schema.bsfs.io/core/Literal/Array/Feature> rdfs:subClassOf bsl:Array .
+            xsd:integer rdfs:subClassOf bsl:Number .
+
+            bsfs:Colors rdfs:subClassOf <https://schema.bsfs.io/core/Literal/Array/Feature> ;
+                bsfs:dimension "5"^^xsd:integer ;
+                bsfs:dtype bsfs:f32 .
+
+            bse:color rdfs:subClassOf bsfs:Predicate ;
+                rdfs:domain bsfs:Node ;
+                rdfs:range bsfs:Colors ;
+                bsfs:unique "true"^^xsd:boolean .
+
+            bse:comment rdfs:subClassOf bsfs:Predicate ;
+                rdfs:domain bsfs:Node ;
+                rdfs:range xsd:string ;
+                bsfs:unique "false"^^xsd:boolean .
+
+            bse:filesize rdfs:subClassOf bsfs:Predicate ;
+                rdfs:domain bsfs:Entity ;
+                rdfs:range xsd:integer ;
+                bsfs:unique "true"^^xsd:boolean .
+
+            bse:tag rdfs:subClassOf bsfs:Predicate ;
+                rdfs:domain bsfs:Entity ;
+                rdfs:range bsfs:Tag ;
+                bsfs:unique "false"^^xsd:boolean .
+
+            bse:label rdfs:subClassOf bsfs:Predicate ;
+                rdfs:domain bsfs:Tag ;
+                rdfs:range xsd:string ;
+                bsfs:unique "true"^^xsd:boolean .
+
+            bse:buddy rdfs:subClassOf bsfs:Predicate ;
+                rdfs:domain bsfs:Entity ;
+                rdfs:range bsfs:Node ;
+                bsfs:unique "false"^^xsd:boolean .
+
+            ''')
+        self.validate = Filter(self.schema)
+
+    def test_call(self): # tests validate implicitly
+        # root_type must be a _schema.Node
+        self.assertRaises(TypeError, self.validate, 1234, None)
+        self.assertRaises(TypeError, self.validate, '1234', None)
+        self.assertRaises(TypeError, self.validate, self.schema.literal(ns.bsfs.URI), None)
+        # root_type must exist in the schema
+        self.assertRaises(errors.ConsistencyError, self.validate, self.schema.node(ns.bsfs.Node).child(ns.bsfs.Image), None)
+        self.assertRaises(errors.ConsistencyError, self.validate, self.schema.node(ns.bsfs.Entity).child(ns.bsfs.Image), None)
+        # valid query returns true
+        self.assertTrue(self.validate(self.schema.node(ns.bsfs.Entity),
+            ast.filter.Any(ast.filter.OneOf(ns.bse.tag, ns.bse.buddy),
+                ast.filter.Or(
+                    ast.filter.Is('http://example.com/symbol#1234'),
+                    ast.filter.All(ns.bse.comment, ast.filter.StartsWith('foo')),
+                    ast.filter.And(
+                        ast.filter.Has(ns.bse.comment, ast.filter.Or(
+                            ast.filter.GreaterThan(5),
+                            ast.filter.LessThan(1),
+                            )
+                        ),
+                        ast.filter.Not(ast.filter.Any(ns.bse.comment,
+                            ast.filter.Not(ast.filter.Equals('hello world')))),
+                    ast.filter.Any(ns.bse.color, ast.filter.Distance([1,2,3,4,5], 3)),
+                    )))))
+        # invalid paths raise consistency error
+        self.assertRaises(errors.ConsistencyError, self.validate, self.schema.node(ns.bsfs.Entity),
+            ast.filter.Any(ast.filter.OneOf(ns.bse.tag, ns.bse.buddy),
+            ast.filter.Or(
+                ast.filter.All(ns.bse.comment, ast.filter.Equals('hello world')),
+                ast.filter.All(ns.bse.label, ast.filter.Equals('hello world')), # domain mismatch
+                )))
+
+    def test_routing(self):
+        self.assertRaises(errors.BackendError, self.validate._parse_filter_expression, ast.filter.FilterExpression(), self.schema.node(ns.bsfs.Node))
+        self.assertRaises(errors.BackendError, self.validate._parse_predicate_expression, ast.filter.PredicateExpression())
+
+    def test_predicate(self):
+        # predicate must be in the schema
+        self.assertRaises(errors.ConsistencyError, self.validate._predicate, ast.filter.Predicate(ns.bse.invalid))
+        # predicate must have a range
+        self.assertRaises(errors.BackendError, self.validate._predicate, ast.filter.Predicate(ns.bsfs.Predicate))
+        # predicate returns domain and range
+        self.assertEqual(self.validate._predicate(ast.filter.Predicate(ns.bse.tag)),
+            (self.schema.node(ns.bsfs.Entity), self.schema.node(ns.bsfs.Tag)))
+        # reverse is applied
+        self.assertEqual(self.validate._predicate(ast.filter.Predicate(ns.bse.tag, reverse=True)),
+            (self.schema.node(ns.bsfs.Tag), self.schema.node(ns.bsfs.Entity)))
+
+    def test_one_of(self):
+        # domains must both be nodes or literals
+        self.assertRaises(errors.ConsistencyError, self.validate._one_of, ast.filter.OneOf(ns.bse.tag, ast.filter.Predicate(ns.bse.label, reverse=True)))
+        # domains must be related
+        self.assertRaises(errors.ConsistencyError, self.validate._one_of, ast.filter.OneOf(ns.bse.tag, ns.bse.label))
+        # ranges must both be nodes or literals
+        self.assertRaises(errors.ConsistencyError, self.validate._one_of, ast.filter.OneOf(ns.bse.tag, ns.bse.comment))
+        # ranges must be related
+        self.assertRaises(errors.ConsistencyError, self.validate._one_of, ast.filter.OneOf(ns.bse.tag, ast.filter.Predicate(ns.bse.buddy, reverse=True)))
+        # one_of returns most specific domain
+        self.assertEqual(self.validate._one_of(ast.filter.OneOf(ns.bse.comment, ns.bse.label)),
+            (self.schema.node(ns.bsfs.Tag), self.schema.literal(ns.xsd.string)))
+        # one_of returns the most generic range
+        self.assertEqual(self.validate._one_of(ast.filter.OneOf(ns.bse.tag, ns.bse.buddy)),
+            (self.schema.node(ns.bsfs.Entity), self.schema.node(ns.bsfs.Node)))
+
+    def test_branch(self):
+        # type must be a node
+        self.assertRaises(errors.ConsistencyError, self.validate._branch, self.schema.literal(ns.bsfs.Literal), None)
+        # type must be in the schema
+        self.assertRaises(errors.ConsistencyError, self.validate._branch, self.schema.node(ns.bsfs.Node).child(ns.bsfs.Invalid), None)
+        # predicate is verified
+        self.assertRaises(errors.ConsistencyError, self.validate._branch, self.schema.node(ns.bsfs.Entity),
+            ast.filter.Any(ns.bsfs.Invalid, ast.filter.Is('http://example.com/entity#1234')))
+        # predicate must match the domain
+        self.assertRaises(errors.ConsistencyError, self.validate._branch, self.schema.node(ns.bsfs.Node),
+            ast.filter.Any(ns.bse.tag, ast.filter.Is('http://example.com/tag#1234')))
+        self.assertRaises(errors.ConsistencyError, self.validate._branch, self.schema.node(ns.bsfs.Tag),
+            ast.filter.Any(ns.bse.tag, ast.filter.Is('http://example.com/tag#1234')))
+        # child expression must be valid
+        self.assertRaises(errors.ConsistencyError, self.validate._branch, self.schema.node(ns.bsfs.Entity),
+            ast.filter.Any(ns.bse.tag, ast.filter.Equals('hello world')))
+        # branch accepts valid expressions
+        self.assertIsNone(self.validate._branch(self.schema.node(ns.bsfs.Entity),
+            ast.filter.Any(ns.bse.tag, ast.filter.Is('http://example.com/entity#1234'))))
+        self.assertIsNone(self.validate._branch(self.schema.node(ns.bsfs.Entity),
+            ast.filter.All(ns.bse.tag, ast.filter.Is('http://example.com/entity#1234'))))
+
+    def test_agg(self):
+        # agg evaluates child expressions
+        self.assertRaises(errors.ConsistencyError, self.validate._agg, self.schema.node(ns.bsfs.Entity),
+            ast.filter.And(ast.filter.Is('http://example.com/entity#1234'), ast.filter.Equals('hello world')))
+        self.assertRaises(errors.ConsistencyError, self.validate._agg, self.schema.literal(ns.xsd.string),
+            ast.filter.And(ast.filter.Is('http://example.com/entity#1234'), ast.filter.Equals('hello world')))
+        self.assertRaises(errors.ConsistencyError, self.validate._agg, self.schema.node(ns.bsfs.Entity),
+            ast.filter.Or(ast.filter.Is('http://example.com/entity#1234'), ast.filter.Equals('hello world')))
+        self.assertRaises(errors.ConsistencyError, self.validate._agg, self.schema.literal(ns.xsd.string),
+            ast.filter.Or(ast.filter.Is('http://example.com/entity#1234'), ast.filter.Equals('hello world')))
+        # agg works on nodes
+        self.assertIsNone(self.validate._agg(self.schema.node(ns.bsfs.Entity),
+            ast.filter.And(ast.filter.Is('http://example.com/entity#1234'), ast.filter.Is('http://example.com/entity#4321'))))
+        self.assertIsNone(self.validate._agg(self.schema.node(ns.bsfs.Entity),
+            ast.filter.Or(ast.filter.Is('http://example.com/entity#1234'), ast.filter.Is('http://example.com/entity#4321'))))
+        # agg works on literals
+        self.assertIsNone(self.validate._agg(self.schema.literal(ns.xsd.string),
+            ast.filter.And(ast.filter.Equals('foobar'), ast.filter.Equals('hello world'))))
+        self.assertIsNone(self.validate._agg(self.schema.literal(ns.xsd.string),
+            ast.filter.Or(ast.filter.Equals('foobar'), ast.filter.Equals('hello world'))))
+
+    def test_not(self):
+        # not evaluates child expressions
+        self.assertRaises(errors.ConsistencyError, self.validate._not, self.schema.node(ns.bsfs.Entity),
+            ast.filter.Not(ast.filter.Equals('hello world')))
+        self.assertRaises(errors.ConsistencyError, self.validate._not, self.schema.literal(ns.xsd.string),
+            ast.filter.Not(ast.filter.Is('http://example.com/entity#1234')))
+        # not works on nodes
+        self.assertIsNone(self.validate._not(self.schema.node(ns.bsfs.Entity),
+            ast.filter.Not(ast.filter.Is('http://example.com/entity#1234'))))
+        # not works on literals
+        self.assertIsNone(self.validate._not(self.schema.literal(ns.xsd.string),
+            ast.filter.Not(ast.filter.Equals('hello world'))))
+
+    def test_has(self):
+        # type must be node
+        self.assertRaises(errors.ConsistencyError, self.validate._has, self.schema.literal(ns.bsfs.Literal),
+            ast.filter.Has(ns.bse.tag))
+        # type must be in the schema
+        self.assertRaises(errors.ConsistencyError, self.validate._has, self.schema.node(ns.bsfs.Node).child(ns.bsfs.Invalid),
+            ast.filter.Has(ns.bse.tag))
+        # has checks predicate
+        self.assertRaises(errors.ConsistencyError, self.validate._has, self.schema.node(ns.bsfs.Entity),
+            ast.filter.Has(ns.bse.invalid))
+        # predicate must match domain
+        self.assertRaises(errors.ConsistencyError, self.validate._has, self.schema.node(ns.bsfs.Tag),
+            ast.filter.Has(ns.bse.tag))
+        # has checks count expression
+        self.assertRaises(errors.ConsistencyError, self.validate._has, self.schema.node(ns.bsfs.Entity),
+            ast.filter.Has(ns.bse.tag, ast.filter.Is('http://example.com/entity#1234')))
+        # has accepts correct expressions
+        self.assertIsNone(self.validate._has(self.schema.node(ns.bsfs.Entity), ast.filter.Has(ns.bse.tag, ast.filter.GreaterThan(5))))
+
+    def test_is(self):
+        # type must be node
+        self.assertRaises(errors.ConsistencyError, self.validate._is, self.schema.literal(ns.bsfs.Literal),
+            ast.filter.Is('http://example.com/foo'))
+        # type must be in the schema
+        self.assertRaises(errors.ConsistencyError, self.validate._is, self.schema.node(ns.bsfs.Node).child(ns.bsfs.Invalid),
+            ast.filter.Is('http://example.com/foo'))
+        # is accepts correct expressions
+        self.assertIsNone(self.validate._is(self.schema.node(ns.bsfs.Entity), ast.filter.Is('http://example.com/entity#1234')))
+
+    def test_value(self):
+        # type must be literal
+        self.assertRaises(errors.ConsistencyError, self.validate._value, self.schema.node(ns.bsfs.Node),
+            ast.filter.Equals('hello world'))
+        self.assertRaises(errors.ConsistencyError, self.validate._value, self.schema.node(ns.bsfs.Node),
+            ast.filter.Substring('hello world'))
+        self.assertRaises(errors.ConsistencyError, self.validate._value, self.schema.node(ns.bsfs.Node),
+            ast.filter.StartsWith('hello world'))
+        self.assertRaises(errors.ConsistencyError, self.validate._value, self.schema.node(ns.bsfs.Node),
+            ast.filter.EndsWith('hello world'))
+        # type must be in the schema
+        self.assertRaises(errors.ConsistencyError, self.validate._value, self.schema.literal(ns.bsfs.Literal).child(ns.bsfs.Invalid),
+            ast.filter.Equals('hello world'))
+        self.assertRaises(errors.ConsistencyError, self.validate._value, self.schema.literal(ns.bsfs.Literal).child(ns.bsfs.Invalid),
+            ast.filter.Substring('hello world'))
+        self.assertRaises(errors.ConsistencyError, self.validate._value, self.schema.literal(ns.bsfs.Literal).child(ns.bsfs.Invalid),
+            ast.filter.StartsWith('hello world'))
+        self.assertRaises(errors.ConsistencyError, self.validate._value, self.schema.literal(ns.bsfs.Literal).child(ns.bsfs.Invalid),
+            ast.filter.EndsWith('hello world'))
+        # value accepts correct expressions
+        self.assertIsNone(self.validate._value(self.schema.literal(ns.xsd.string), ast.filter.Equals('hello world')))
+        self.assertIsNone(self.validate._value(self.schema.literal(ns.xsd.string), ast.filter.Substring('hello world')))
+        self.assertIsNone(self.validate._value(self.schema.literal(ns.xsd.string), ast.filter.StartsWith('hello world')))
+        self.assertIsNone(self.validate._value(self.schema.literal(ns.xsd.string), ast.filter.EndsWith('hello world')))
+
+    def test_bounded(self):
+        # type must be literal
+        self.assertRaises(errors.ConsistencyError, self.validate._bounded, self.schema.node(ns.bsfs.Node),
+            ast.filter.GreaterThan(0))
+        self.assertRaises(errors.ConsistencyError, self.validate._bounded, self.schema.node(ns.bsfs.Node),
+            ast.filter.LessThan(0))
+        # type must be in the schema
+        self.assertRaises(errors.ConsistencyError, self.validate._bounded, self.schema.literal(ns.bsfs.Literal).child(ns.bsfs.Invalid),
+            ast.filter.GreaterThan(0))
+        self.assertRaises(errors.ConsistencyError, self.validate._bounded, self.schema.literal(ns.bsfs.Literal).child(ns.bsfs.Invalid),
+            ast.filter.LessThan(0))
+        # type must be a number
+        self.assertRaises(errors.ConsistencyError, self.validate._bounded, self.schema.literal(ns.xsd.string),
+            ast.filter.LessThan(0))
+        # bounded accepts correct expressions
+        self.assertIsNone(self.validate._bounded(self.schema.literal(ns.xsd.integer), ast.filter.LessThan(0)))
+        self.assertIsNone(self.validate._bounded(self.schema.literal(ns.xsd.integer), ast.filter.GreaterThan(0)))
+
+    def test_distance(self):
+        # type must be a literal
+        self.assertRaises(errors.ConsistencyError, self.validate._distance, self.schema.node(ns.bsfs.Node),
+            ast.filter.Distance([1,2,3], 1, False))
+        # type must be a feature
+        self.assertRaises(errors.ConsistencyError, self.validate._distance, self.schema.literal(ns.bsl.Array),
+            ast.filter.Distance([1,2,3], 1, False))
+        # type must be in the schema
+        self.assertRaises(errors.ConsistencyError, self.validate._distance, self.schema.literal(ns.bsl.Array.Feature).child(ns.bsfs.Invalid),
+            ast.filter.Distance([1,2,3], 1, False))
+        # FIXME: reference must be a numpy array
+        # reference must have the correct dimension
+        self.assertRaises(errors.ConsistencyError, self.validate._distance, self.schema.literal(ns.bsfs.Colors),
+            ast.filter.Distance([1,2,3], 1, False))
+        # FIXME: reference must have the correct dtype
+        # distance accepts correct expressions
+        self.assertIsNone(self.validate._distance(self.schema.literal(ns.bsfs.Colors), ast.filter.Distance([1,2,3,4,5], 1, False)))
+
+
+class TestFetch(unittest.TestCase):
+    def setUp(self):
+        self.schema = _schema.from_string('''
+            prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
+            prefix xsd: <http://www.w3.org/2001/XMLSchema#>
+
+            prefix bsfs: <https://schema.bsfs.io/core/>
+            prefix bse: <https://schema.bsfs.io/core/Entity#>
+
+            bsfs:Entity rdfs:subClassOf bsfs:Node .
+            bsfs:Tag rdfs:subClassOf bsfs:Node .
+            xsd:string rdfs:subClassOf bsfs:Literal .
+
+            bse:filename rdfs:subClassOf bsfs:Predicate ;
+                rdfs:domain bsfs:Node ;
+                rdfs:range xsd:string .
+
+            bse:tag rdfs:subClassOf bsfs:Predicate ;
+                rdfs:domain bsfs:Entity ;
+                rdfs:range bsfs:Tag .
+
+            bse:label rdfs:subClassOf bsfs:Predicate ;
+                rdfs:domain bsfs:Tag ;
+                rdfs:range xsd:string .
+
+            ''')
+        self.validate = Fetch(self.schema)
+
+    def test_call(self): # tests validate implicitly
+        # call accepts correct expressions
+        self.assertTrue(self.validate(self.schema.node(ns.bsfs.Entity),
+            ast.fetch.Fetch(ns.bse.tag, ast.fetch.Value(ns.bse.label, 'value'))))
+        self.assertTrue(self.validate(self.schema.node(ns.bsfs.Entity),
+            ast.fetch.Fetch(ns.bse.tag, ast.fetch.This('this'))))
+        self.assertTrue(self.validate(self.schema.node(ns.bsfs.Entity),
+            ast.fetch.This('this')))
+        self.assertTrue(self.validate(self.schema.node(ns.bsfs.Entity),
+            ast.fetch.All(ast.fetch.This('this'), ast.fetch.Node(ns.bse.tag, 'node'), ast.fetch.Value(ns.bse.filename, 'value'))))
+        # type must be a Node
+        self.assertRaises(TypeError, self.validate, 1234, ast.fetch.This('this'))
+        self.assertRaises(TypeError, self.validate, 'foobar', ast.fetch.This('this'))
+        self.assertRaises(TypeError, self.validate, self.schema.literal(ns.bsfs.Literal), ast.fetch.This('this'))
+        # type must be in the schema
+        self.assertRaises(errors.ConsistencyError, self.validate, self.schema.node(ns.bsfs.Node).child(ns.bsfs.Invalid),
+            ast.fetch.FetchExpression())
+        # expression must be a fetch expression
+        self.assertRaises(TypeError, self.validate, self.schema.node(ns.bsfs.Entity), 1234)
+        self.assertRaises(TypeError, self.validate, self.schema.node(ns.bsfs.Entity), 'hello')
+        self.assertRaises(TypeError, self.validate, self.schema.node(ns.bsfs.Entity), ast.filter.FilterExpression())
+        # expression must be valid
+        self.assertRaises(errors.ConsistencyError, self.validate, self.schema.node(ns.bsfs.Entity),
+            ast.fetch.Fetch(ns.bse.tag, ast.fetch.Node(ns.bse.label, 'node')))
+        self.assertRaises(errors.ConsistencyError, self.validate, self.schema.node(ns.bsfs.Entity),
+            ast.fetch.Value(ns.bse.tag, 'value'))
+
+    def test_routing(self):
+        # Node passes _branch, _named, and _node checks
+        self.assertRaises(errors.ConsistencyError, self.validate._parse_fetch_expression, self.schema.node(ns.bsfs.Node),
+            ast.fetch.Node(ns.bse.tag, 'node')) # fails in _branch
+        self.assertRaises(errors.BackendError, self.validate._parse_fetch_expression, self.schema.node(ns.bsfs.Entity),
+            ast.fetch.Node(ns.bse.tag, '')) # fails in _named
+        self.assertRaises(errors.ConsistencyError, self.validate._parse_fetch_expression, self.schema.node(ns.bsfs.Entity),
+            ast.fetch.Node(ns.bse.label, 'node')) # fails in _node
+        # Value passes _branch, _named, and _value checks
+        self.assertRaises(errors.ConsistencyError, self.validate._parse_fetch_expression, self.schema.node(ns.bsfs.Node),
+            ast.fetch.Value(ns.bse.label, 'value')) # fails in _branch
+        self.assertRaises(errors.BackendError, self.validate._parse_fetch_expression, self.schema.node(ns.bsfs.Entity),
+            ast.fetch.Value(ns.bse.filename, '')) # fails in _named
+        self.assertRaises(errors.ConsistencyError, self.validate._parse_fetch_expression, self.schema.node(ns.bsfs.Entity),
+            ast.fetch.Value(ns.bse.tag, 'value')) # fails in _value
+        # Fetch passes _branch and _fetch checks
+        self.assertRaises(errors.ConsistencyError, self.validate._parse_fetch_expression, self.schema.node(ns.bsfs.Node),
+            ast.fetch.Fetch(ns.bse.tag, ast.fetch.This('this'))) # fails in _branch
+        self.assertRaises(errors.ConsistencyError, self.validate._parse_fetch_expression, self.schema.node(ns.bsfs.Node),
+            ast.fetch.Fetch(ns.bse.filename, ast.fetch.This('this'))) # fails in _fetch
+        # invalid expressions cannot be parsed
+        type_ = self.schema.node(ns.bsfs.Node)
+        self.assertRaises(errors.BackendError, self.validate._parse_fetch_expression, type_,
+            ast.filter.FilterExpression())
+        self.assertRaises(errors.BackendError, self.validate._parse_fetch_expression, type_,
+            1234)
+        self.assertRaises(errors.BackendError, self.validate._parse_fetch_expression, type_,
+            'hello world')
+
+    def test_all(self):
+        # all accepts correct expressions
+        self.assertIsNone(self.validate._all(self.schema.node(ns.bsfs.Entity),
+            ast.fetch.All(ast.fetch.Value(ns.bse.filename, 'value'), ast.fetch.Node(ns.bse.tag, 'node'))))
+        # child expressions must be valid
+        self.assertRaises(errors.ConsistencyError, self.validate._all, self.schema.node(ns.bsfs.Entity),
+            ast.fetch.All(ast.fetch.Value(ns.bse.tag, 'value')))
+        self.assertRaises(errors.ConsistencyError, self.validate._all, self.schema.node(ns.bsfs.Entity),
+            ast.fetch.All(ast.fetch.Value(ns.bse.filename, 'value'), ast.fetch.Node(ns.bse.filename, 'node')))
+        self.assertRaises(errors.ConsistencyError, self.validate._all, self.schema.node(ns.bsfs.Entity),
+            ast.fetch.All(ast.fetch.Value(ns.bse.tag, 'value'), ast.fetch.Node(ns.bse.tag, 'node')))
+        self.assertRaises(errors.ConsistencyError, self.validate._all, self.schema.node(ns.bsfs.Entity),
+            ast.fetch.All(ast.fetch.Value(ns.bse.tag, 'value'), ast.fetch.Node(ns.bse.filename, 'node')))
+
+    def test_branch(self):
+        # branch accepts correct expressions
+        self.assertIsNone(self.validate._branch(self.schema.node(ns.bsfs.Entity),
+            ast.fetch._Branch(ns.bse.filename)))
+        self.assertIsNone(self.validate._branch(self.schema.node(ns.bsfs.Entity),
+            ast.fetch.Fetch(ns.bse.tag, ast.fetch.This('this'))))
+        self.assertIsNone(self.validate._branch(self.schema.node(ns.bsfs.Entity),
+            ast.fetch.Value(ns.bse.filename, 'value')))
+        self.assertIsNone(self.validate._branch(self.schema.node(ns.bsfs.Entity),
+            ast.fetch.Node(ns.bse.tag, 'node')))
+        # type must be a node
+        self.assertRaises(errors.ConsistencyError, self.validate._branch, self.schema.literal(ns.bsfs.Literal),
+            ast.fetch._Branch(ns.bse.filename))
+        self.assertRaises(errors.ConsistencyError, self.validate._branch, self.schema.literal(ns.bsfs.Literal),
+            ast.fetch.Fetch(ns.bse.tag, ast.fetch.This('this')))
+        self.assertRaises(errors.ConsistencyError, self.validate._branch, self.schema.literal(ns.bsfs.Literal),
+            ast.fetch.Value(ns.bse.filename, 'value'))
+        self.assertRaises(errors.ConsistencyError, self.validate._branch, self.schema.literal(ns.bsfs.Literal),
+            ast.fetch.Node(ns.bse.tag, 'node'))
+        # type must be in the schema
+        self.assertRaises(errors.ConsistencyError, self.validate._branch, self.schema.node(ns.bsfs.Node).child(ns.bsfs.Invalid),
+            ast.fetch._Branch(ns.bse.filename))
+        self.assertRaises(errors.ConsistencyError, self.validate._branch, self.schema.node(ns.bsfs.Node).child(ns.bsfs.Invalid),
+            ast.fetch.Fetch(ns.bse.tag, ast.fetch.This('this')))
+        self.assertRaises(errors.ConsistencyError, self.validate._branch, self.schema.node(ns.bsfs.Node).child(ns.bsfs.Invalid),
+            ast.fetch.Value(ns.bse.filename, 'value'))
+        self.assertRaises(errors.ConsistencyError, self.validate._branch, self.schema.node(ns.bsfs.Node).child(ns.bsfs.Invalid),
+            ast.fetch.Node(ns.bse.tag, 'node'))
+        # predicate must be in the schema
+        self.assertRaises(errors.ConsistencyError, self.validate._branch, self.schema.node(ns.bsfs.Entity),
+            ast.fetch._Branch(ns.bse.invalid))
+        self.assertRaises(errors.ConsistencyError, self.validate._branch, self.schema.node(ns.bsfs.Entity),
+            ast.fetch.Fetch(ns.bse.invalid, ast.fetch.This('this')))
+        self.assertRaises(errors.ConsistencyError, self.validate._branch, self.schema.node(ns.bsfs.Entity),
+            ast.fetch.Value(ns.bse.invalid, 'value'))
+        self.assertRaises(errors.ConsistencyError, self.validate._branch, self.schema.node(ns.bsfs.Entity),
+            ast.fetch.Node(ns.bse.invalid, 'node'))
+        # predicate's domain must be related to the type
+        self.assertRaises(errors.ConsistencyError, self.validate._branch, self.schema.node(ns.bsfs.Entity),
+            ast.fetch._Branch(ns.bse.label))
+        self.assertRaises(errors.ConsistencyError, self.validate._branch, self.schema.node(ns.bsfs.Entity),
+            ast.fetch.Fetch(ns.bse.label, ast.fetch.This('this')))
+        self.assertRaises(errors.ConsistencyError, self.validate._branch, self.schema.node(ns.bsfs.Entity),
+            ast.fetch.Node(ns.bse.label, 'node'))
+        self.assertRaises(errors.ConsistencyError, self.validate._branch, self.schema.node(ns.bsfs.Entity),
+            ast.fetch.Value(ns.bse.label, 'value'))
+        # predicate's domain cannot be a supertype
+        self.assertRaises(errors.ConsistencyError, self.validate._branch, self.schema.node(ns.bsfs.Node),
+            ast.fetch._Branch(ns.bse.tag))
+        self.assertRaises(errors.ConsistencyError, self.validate._branch, self.schema.node(ns.bsfs.Node),
+            ast.fetch.Fetch(ns.bse.tag, ast.fetch.This('this')))
+        self.assertRaises(errors.ConsistencyError, self.validate._branch, self.schema.node(ns.bsfs.Node),
+            ast.fetch.Node(ns.bse.tag, 'node'))
+        self.assertRaises(errors.ConsistencyError, self.validate._branch, self.schema.node(ns.bsfs.Node),
+            ast.fetch.Value(ns.bse.tag, 'value'))
+        # predicate's domain can be a subtype
+        self.assertIsNone(self.validate._branch(self.schema.node(ns.bsfs.Entity),
+            ast.fetch._Branch(ns.bse.filename)))
+        self.assertIsNone(self.validate._branch(self.schema.node(ns.bsfs.Entity),
+            ast.fetch.Value(ns.bse.filename, 'value')))
+
+    def test_fetch(self):
+        # fetch accepts correct expressions
+        self.assertIsNone(self.validate._fetch(self.schema.node(ns.bsfs.Node),
+            ast.fetch.Fetch(ns.bse.tag, ast.fetch.Value(ns.bse.label, 'value'))))
+        # range must be a node
+        self.assertRaises(errors.ConsistencyError, self.validate._fetch, self.schema.node(ns.bsfs.Node),
+            ast.fetch.Fetch(ns.bse.filename, ast.fetch.This('this')))
+        # child expression must be valid
+        self.assertRaises(errors.ConsistencyError, self.validate._fetch, self.schema.node(ns.bsfs.Node),
+            ast.fetch.Fetch(ns.bse.tag, ast.fetch.Node(ns.bse.label, 'node')))
+
+    def test_named(self):
+        # named accepts correct expressions
+        self.assertIsNone(self.validate._named(self.schema.node(ns.bsfs.Entity),
+            ast.fetch.Node(ns.bse.tag, 'node')))
+        self.assertIsNone(self.validate._named(self.schema.node(ns.bsfs.Entity),
+            ast.fetch.Value(ns.bse.filename, 'value')))
+        # name must be non-empty
+        self.assertRaises(errors.BackendError, self.validate._named, self.schema.node(ns.bsfs.Entity),
+            ast.fetch.Node(ns.bse.tag, ''))
+        self.assertRaises(errors.BackendError, self.validate._named, self.schema.node(ns.bsfs.Entity),
+            ast.fetch.Value(ns.bse.filename, ''))
+
+    def test_node(self):
+        # node accepts correct expressions
+        self.assertIsNone(self.validate._node(self.schema.node(ns.bsfs.Entity),
+            ast.fetch.Node(ns.bse.tag, 'node')))
+        # range must be a node
+        self.assertRaises(errors.ConsistencyError, self.validate._node, self.schema.node(ns.bsfs.Entity),
+            ast.fetch.Node(ns.bse.filename, 'node'))
+
+    def test_value(self):
+        # value accepts correct expressions
+        self.assertIsNone(self.validate._value(self.schema.node(ns.bsfs.Entity),
+            ast.fetch.Value(ns.bse.filename, 'value')))
+        # range must be a literal
+        self.assertRaises(errors.ConsistencyError, self.validate._value, self.schema.node(ns.bsfs.Entity),
+            ast.fetch.Value(ns.bse.tag, 'value'))
+
+    def test_this(self):
+        # this accepts correct expressions
+        self.assertIsNone(self.validate._this(self.schema.node(ns.bsfs.Entity), ast.fetch.This('this')))
+        # type must be a node
+        self.assertRaises(errors.ConsistencyError, self.validate._this, self.schema.literal(ns.bsfs.Literal),
+            ast.fetch.This('this'))
+        self.assertRaises(errors.ConsistencyError, self.validate._this, self.schema.predicate(ns.bsfs.Predicate),
+            ast.fetch.This('this'))
+        # type must be in the schema
+        self.assertRaises(errors.ConsistencyError, self.validate._this, self.schema.node(ns.bsfs.Node).child(ns.bsfs.Invalid),
+            ast.fetch.This('this'))
+        # name must be non-empty
+        self.assertRaises(errors.BackendError, self.validate._this, self.schema.node(ns.bsfs.Entity), ast.fetch.This(''))
+
+
+## main ##
+
+if __name__ == '__main__':
+    unittest.main()
+
+## EOF ##
diff --git a/test/schema/test_schema.py b/test/schema/test_schema.py
index 888cdca..f52cf95 100644
--- a/test/schema/test_schema.py
+++ b/test/schema/test_schema.py
@@ -1,16 +1,11 @@
-"""
 
-Part of the tagit test suite.
-A copy of the license is provided with the project.
-Author: Matthias Baumgartner, 2022
-"""
 # imports
 import operator
 import unittest
 
 # bsfs imports
 from bsfs.namespace import ns
-from bsfs.schema import types
+from bsfs.schema import types, from_string
 from bsfs.utils import errors
 
 # objects to test
@@ -19,6 +14,8 @@ from bsfs.schema.schema import Schema
 
 ## code ##
 
+ns.bse = ns.bsfs.Entity()
+
 class TestSchema(unittest.TestCase):
 
     def setUp(self):
@@ -26,8 +23,9 @@ class TestSchema(unittest.TestCase):
             prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
             prefix xsd: <http://www.w3.org/2001/XMLSchema#>
 
-            prefix bsfs: <http://bsfs.ai/schema/>
-            prefix bse: <http://bsfs.ai/schema/Entity#>
+            prefix bsfs: <https://schema.bsfs.io/core/>
+            prefix bse: <https://schema.bsfs.io/core/Entity#>
+            prefix bsl: <https://schema.bsfs.io/core/Literal>
 
             bsfs:Entity rdfs:subClassOf bsfs:Node .
             bsfs:Tag rdfs:subClassOf bsfs:Node .
@@ -35,7 +33,8 @@ class TestSchema(unittest.TestCase):
             bsfs:Unused rdfs:subClassOf bsfs:Node .
 
             xsd:string rdfs:subClassOf bsfs:Literal .
-            xsd:integer rdfs:subClassOf bsfs:Literal .
+            bsfs:Number rdfs:subClassOf bsfs:Literal .
+            xsd:integer rdfs:subClassOf bsl:Number .
             xsd:boolean rdfs:subClassOf bsfs:Literal .
 
             bse:tag rdfs:subClassOf bsfs:Predicate ;
@@ -55,32 +54,43 @@ class TestSchema(unittest.TestCase):
 
             '''
         # nodes
-        self.n_root = types.Node(ns.bsfs.Node, None)
-        self.n_ent = types.Node(ns.bsfs.Entity, types.Node(ns.bsfs.Node, None))
-        self.n_img = types.Node(ns.bsfs.Image, types.Node(ns.bsfs.Entity, types.Node(ns.bsfs.Node, None)))
-        self.n_tag = types.Node(ns.bsfs.Tag, types.Node(ns.bsfs.Node, None))
-        self.n_unused = types.Node(ns.bsfs.Unused, types.Node(ns.bsfs.Node, None))
+        self.n_root = types.ROOT_NODE
+        self.n_ent = self.n_root.child(ns.bsfs.Entity)
+        self.n_img = self.n_ent.child(ns.bsfs.Image)
+        self.n_tag = self.n_root.child(ns.bsfs.Tag)
+        self.n_unused = self.n_root.child(ns.bsfs.Unused)
         self.nodes = [self.n_root, self.n_ent, self.n_img, self.n_tag, self.n_unused]
 
         # literals
-        self.l_root = types.Literal(ns.bsfs.Literal, None)
-        self.l_string = types.Literal(ns.xsd.string, types.Literal(ns.bsfs.Literal, None))
-        self.l_integer = types.Literal(ns.xsd.integer, types.Literal(ns.bsfs.Literal, None))
-        self.l_unused = types.Literal(ns.xsd.boolean, types.Literal(ns.bsfs.Literal, None))
-        self.literals = [self.l_root, self.l_string, self.l_integer, self.l_unused]
+        self.l_root = types.ROOT_LITERAL
+        self.l_number = types.ROOT_NUMBER
+        self.l_blob = types.ROOT_BLOB
+        self.l_array = types.ROOT_ARRAY
+        self.l_time = types.ROOT_TIME
+        self.l_string = self.l_root.child(ns.xsd.string)
+        self.l_integer = self.l_root.child(ns.xsd.integer)
+        self.l_unused = self.l_root.child(ns.xsd.boolean)
+        self.f_root = types.ROOT_FEATURE
+        self.literals = [self.l_root, self.l_array, self.f_root, self.l_number, self.l_time, self.l_string, self.l_integer, self.l_unused, self.l_blob]
 
         # predicates
-        self.p_root = types.Predicate(ns.bsfs.Predicate, None, types.Node(ns.bsfs.Node, None), None, False)
-        self.p_tag = self.p_root.get_child(ns.bse.tag, self.n_ent, self.n_tag, False)
-        self.p_group = self.p_tag.get_child(ns.bse.group, self.n_img, self.n_tag, False)
-        self.p_comment = self.p_root.get_child(ns.bse.comment, self.n_root, self.l_string, True)
+        self.p_root = types.ROOT_PREDICATE
+        self.p_tag = self.p_root.child(ns.bse.tag, self.n_ent, self.n_tag, False)
+        self.p_group = self.p_tag.child(ns.bse.group, self.n_img, self.n_tag, False)
+        self.p_comment = self.p_root.child(ns.bse.comment, self.n_root, self.l_string, True)
         self.predicates = [self.p_root, self.p_tag, self.p_group, self.p_comment]
 
     def test_construction(self):
+        # no args yields a minimal schema
+        schema = Schema()
+        self.assertSetEqual(set(schema.nodes()), {self.n_root})
+        self.assertSetEqual(set(schema.literals()), {self.l_root, self.l_number, self.l_array, self.l_time, self.f_root, self.l_blob})
+        self.assertSetEqual(set(schema.predicates()), {self.p_root})
+
         # nodes and literals are optional
         schema = Schema(self.predicates)
         self.assertSetEqual(set(schema.nodes()), {self.n_root, self.n_ent, self.n_img, self.n_tag})
-        self.assertSetEqual(set(schema.literals()), {self.l_root, self.l_string})
+        self.assertSetEqual(set(schema.literals()), {self.l_root, self.l_string, self.l_number, self.l_time, self.l_array, self.f_root, self.l_blob})
         self.assertSetEqual(set(schema.predicates()), set(self.predicates))
 
         # predicates, nodes, and literals are respected
@@ -101,19 +111,19 @@ class TestSchema(unittest.TestCase):
 
         # literals are complete
         schema = Schema(self.predicates, self.nodes, None)
-        self.assertSetEqual(set(schema.literals()), {self.l_root, self.l_string})
+        self.assertSetEqual(set(schema.literals()), {self.l_root, self.l_string, self.l_number, self.l_array, self.l_time, self.f_root, self.l_blob})
         schema = Schema(self.predicates, self.nodes, [])
-        self.assertSetEqual(set(schema.literals()), {self.l_root, self.l_string})
+        self.assertSetEqual(set(schema.literals()), {self.l_root, self.l_string, self.l_number, self.l_array, self.l_time, self.f_root, self.l_blob})
         schema = Schema(self.predicates, self.nodes, [self.l_string])
-        self.assertSetEqual(set(schema.literals()), {self.l_root, self.l_string})
+        self.assertSetEqual(set(schema.literals()), {self.l_root, self.l_string, self.l_number, self.l_array, self.l_time, self.f_root, self.l_blob})
         schema = Schema(self.predicates, self.nodes, [self.l_integer])
-        self.assertSetEqual(set(schema.literals()), {self.l_root, self.l_string, self.l_integer})
+        self.assertSetEqual(set(schema.literals()), {self.l_root, self.l_string, self.l_integer, self.l_number, self.l_array, self.l_time, self.f_root, self.l_blob})
         schema = Schema(self.predicates, self.nodes, [self.l_integer, self.l_unused])
         self.assertSetEqual(set(schema.literals()), set(self.literals))
 
         # predicates are complete
         schema = Schema([], self.nodes, self.literals)
-        self.assertSetEqual(set(schema.predicates()), set())
+        self.assertSetEqual(set(schema.predicates()), {self.p_root})
         schema = Schema([self.p_group], self.nodes, self.literals)
         self.assertSetEqual(set(schema.predicates()), {self.p_root, self.p_tag, self.p_group})
         schema = Schema([self.p_group, self.p_comment], self.nodes, self.literals)
@@ -153,20 +163,27 @@ class TestSchema(unittest.TestCase):
         self.assertRaises(errors.ConsistencyError, Schema,
             {}, {types.Node(ns.bsfs.Foo, None)}, {types.Node(ns.bsfs.Foo, None)})
         self.assertRaises(errors.ConsistencyError, Schema,
-            {types.Predicate(ns.bsfs.Foo, None, types.Node(ns.bsfs.Node, None), None, False)}, {}, {types.Node(ns.bsfs.Foo, None)})
+            {types.Predicate(ns.bsfs.Foo, None, types.Node(ns.bsfs.Node, None), types.ROOT_VERTEX, False)}, {}, {types.Node(ns.bsfs.Foo, None)})
         self.assertRaises(errors.ConsistencyError, Schema,
-            {types.Predicate(ns.bsfs.Foo, None, types.Node(ns.bsfs.Node, None), None, False)}, {types.Node(ns.bsfs.Foo, None)}, {})
+            {types.Predicate(ns.bsfs.Foo, None, types.Node(ns.bsfs.Node, None), types.ROOT_VERTEX, False)}, {types.Node(ns.bsfs.Foo, None)}, {})
         self.assertRaises(errors.ConsistencyError, Schema,
-            {types.Predicate(ns.bsfs.Foo, None, types.Node(ns.bsfs.Node, None), None, False)}, {types.Node(ns.bsfs.Foo, None)}, {types.Node(ns.bsfs.Foo, None)})
+            {types.Predicate(ns.bsfs.Foo, None, types.Node(ns.bsfs.Node, None), types.ROOT_VERTEX, False)}, {types.Node(ns.bsfs.Foo, None)}, {types.Node(ns.bsfs.Foo, None)})
+
 
     def test_str(self):
+        # string conversion
         self.assertEqual(str(Schema([])), 'Schema()')
         self.assertEqual(str(Schema([], [], [])), 'Schema()')
         self.assertEqual(str(Schema(self.predicates, self.nodes, self.literals)), 'Schema()')
-        self.assertEqual(repr(Schema([])), 'Schema([], [], [])')
-        self.assertEqual(repr(Schema([], [], [])), 'Schema([], [], [])')
+        # repr conversion with only default nodes, literals, and predicates
+        n = [ns.bsfs.Node]
+        l = [ns.bsfs.Literal, ns.bsl.Array, ns.bsl.Array.Feature, ns.bsl.BinaryBlob, ns.bsl.Number, ns.bsl.Time]
+        p = [ns.bsfs.Predicate]
+        self.assertEqual(repr(Schema()), f'Schema({n}, {l}, {p})')
+        self.assertEqual(repr(Schema([], [], [])), f'Schema({n}, {l}, {p})')
+        # repr conversion
         n = [ns.bsfs.Entity, ns.bsfs.Image, ns.bsfs.Node, ns.bsfs.Tag, ns.bsfs.Unused]
-        l = [ns.bsfs.Literal, ns.xsd.boolean, ns.xsd.integer, ns.xsd.string]
+        l = [ns.xsd.boolean, ns.xsd.integer, ns.xsd.string, ns.bsfs.Literal, ns.bsl.Array, ns.bsl.Array.Feature, ns.bsl.BinaryBlob, ns.bsl.Number, ns.bsl.Time]
         p = [ns.bse.comment, ns.bse.group, ns.bse.tag, ns.bsfs.Predicate]
         self.assertEqual(repr(Schema(self.predicates, self.nodes, self.literals)), f'Schema({n}, {l}, {p})')
 
@@ -202,16 +219,16 @@ class TestSchema(unittest.TestCase):
         self.assertNotEqual(hash(schema),
             hash(Schema([self.p_group, self.p_tag, self.p_root], self.nodes, self.literals)))
         self.assertNotEqual(schema,
-            Schema(self.predicates + [self.p_root.get_child(ns.bse.filesize, self.n_ent, self.l_integer)], self.nodes, self.literals))
+            Schema(self.predicates + [self.p_root.child(ns.bse.filesize, self.n_ent, self.l_integer)], self.nodes, self.literals))
         self.assertNotEqual(hash(schema),
-            hash(Schema(self.predicates + [self.p_root.get_child(ns.bse.filesize, self.n_ent, self.l_integer)], self.nodes, self.literals)))
+            hash(Schema(self.predicates + [self.p_root.child(ns.bse.filesize, self.n_ent, self.l_integer)], self.nodes, self.literals)))
 
     def test_order(self):
         # setup
         class Foo(): pass
-        p_foo = self.p_root.get_child(ns.bse.foo, self.n_ent, self.l_string, True)
-        p_sub = p_foo.get_child(ns.bse.sub, self.n_ent, self.l_string, True)
-        p_bar = self.p_root.get_child(ns.bse.bar, self.n_ent, self.l_string, True)
+        p_foo = self.p_root.child(ns.bse.foo, self.n_ent, self.l_string, True)
+        p_sub = p_foo.child(ns.bse.sub, self.n_ent, self.l_string, True)
+        p_bar = self.p_root.child(ns.bse.bar, self.n_ent, self.l_string, True)
 
         # can only compare schema to other schema
         # <
@@ -258,11 +275,11 @@ class TestSchema(unittest.TestCase):
         self.assertTrue(operator.lt(Schema({self.p_tag}), Schema({self.p_group})))
         self.assertTrue(operator.le(Schema({self.p_tag}), Schema({self.p_group})))
         # subset considers differences in predicates and literals
-        self.assertTrue(operator.lt(Schema.Empty(), Schema({self.p_comment})))
+        self.assertTrue(operator.lt(Schema(), Schema({self.p_comment})))
         # subset considers differences in predicates, nodes, and literals
-        self.assertTrue(operator.lt(Schema({}), Schema.Empty()))
-        self.assertTrue(operator.lt(Schema({self.p_tag}), Schema.from_string(self.schema_str)))
-        self.assertTrue(operator.le(Schema({self.p_tag}), Schema.from_string(self.schema_str)))
+        self.assertTrue(operator.le(Schema({}), Schema()))
+        self.assertTrue(operator.lt(Schema({self.p_tag}), from_string(self.schema_str)))
+        self.assertTrue(operator.le(Schema({self.p_tag}), from_string(self.schema_str)))
         self.assertFalse(operator.lt(Schema({self.p_comment}), Schema({self.p_tag})))
         self.assertFalse(operator.le(Schema({self.p_comment}), Schema({self.p_tag})))
 
@@ -280,54 +297,54 @@ class TestSchema(unittest.TestCase):
         self.assertTrue(operator.gt(Schema({self.p_group}), Schema({self.p_tag})))
         self.assertTrue(operator.ge(Schema({self.p_group}), Schema({self.p_tag})))
         # superset considers differences in predicates and literals
-        self.assertTrue(operator.gt(Schema({self.p_comment}), Schema.Empty()))
+        self.assertTrue(operator.gt(Schema({self.p_comment}), Schema()))
         # superset considers differences in predicates, nodes, and literals
-        self.assertTrue(operator.gt(Schema.Empty(), Schema({})))
-        self.assertTrue(operator.gt(Schema.from_string(self.schema_str), Schema({self.p_tag})))
-        self.assertTrue(operator.ge(Schema.from_string(self.schema_str), Schema({self.p_tag})))
+        self.assertTrue(operator.ge(Schema(), Schema({})))
+        self.assertTrue(operator.gt(from_string(self.schema_str), Schema({self.p_tag})))
+        self.assertTrue(operator.ge(from_string(self.schema_str), Schema({self.p_tag})))
         self.assertFalse(operator.gt(Schema({self.p_tag}), Schema({self.p_comment})))
         self.assertFalse(operator.ge(Schema({self.p_tag}), Schema({self.p_comment})))
 
         # inconsistent schema cannot be a subset
         self.assertFalse(operator.le(Schema({p_foo}), Schema({
-            self.p_root.get_child(ns.bse.foo, self.n_ent, self.l_integer, True)}))) # inconsistent w.r.t. literal
+            self.p_root.child(ns.bse.foo, self.n_ent, self.l_integer, True)}))) # inconsistent w.r.t. literal
         self.assertFalse(operator.le(Schema({p_foo}), Schema({
-            self.p_root.get_child(ns.bse.foo, self.n_img, self.l_string, True)}))) # inconsistent w.r.t. node
+            self.p_root.child(ns.bse.foo, self.n_img, self.l_string, True)}))) # inconsistent w.r.t. node
         self.assertFalse(operator.le(Schema({p_foo}), Schema({
-            self.p_root.get_child(ns.bse.foo, self.n_ent, self.l_string, False)}))) # inconsistent w.r.t. unique
+            self.p_root.child(ns.bse.foo, self.n_ent, self.l_string, False)}))) # inconsistent w.r.t. unique
         self.assertFalse(operator.le(Schema({}, {self.n_img}), Schema({}, {
             types.Node(ns.bsfs.Image, types.Node(ns.bsfs.Node, None))})))
         self.assertFalse(operator.le(Schema({}, {}, {self.l_integer}), Schema({}, {}, {
             types.Literal(ns.xsd.integer, types.Literal(ns.xsd.number, types.Literal(ns.bsfs.Literal, None)))})))
         # inconsistent schema cannot be a true subset
         self.assertFalse(operator.lt(Schema({p_foo}), Schema({
-            self.p_root.get_child(ns.bse.foo, self.n_ent, self.l_integer, True)}))) # inconsistent w.r.t. literal
+            self.p_root.child(ns.bse.foo, self.n_ent, self.l_integer, True)}))) # inconsistent w.r.t. literal
         self.assertFalse(operator.lt(Schema({p_foo}), Schema({
-            self.p_root.get_child(ns.bse.foo, self.n_img, self.l_string, True)}))) # inconsistent w.r.t. node
+            self.p_root.child(ns.bse.foo, self.n_img, self.l_string, True)}))) # inconsistent w.r.t. node
         self.assertFalse(operator.lt(Schema({p_foo}), Schema({
-            self.p_root.get_child(ns.bse.foo, self.n_ent, self.l_string, False)}))) # inconsistent w.r.t. unique
+            self.p_root.child(ns.bse.foo, self.n_ent, self.l_string, False)}))) # inconsistent w.r.t. unique
         self.assertFalse(operator.lt(Schema({}, {self.n_img}), Schema({}, {
             types.Node(ns.bsfs.Image, types.Node(ns.bsfs.Node, None))})))
         self.assertFalse(operator.lt(Schema({}, {}, {self.l_integer}), Schema({}, {}, {
             types.Literal(ns.xsd.integer, types.Literal(ns.xsd.number, types.Literal(ns.bsfs.Literal, None)))})))
         # inconsistent schema cannot be a superset
         self.assertFalse(operator.ge(Schema({p_foo}), Schema({
-            self.p_root.get_child(ns.bse.foo, self.n_ent, self.l_integer, True)}))) # inconsistent w.r.t. literal
+            self.p_root.child(ns.bse.foo, self.n_ent, self.l_integer, True)}))) # inconsistent w.r.t. literal
         self.assertFalse(operator.ge(Schema({p_foo}), Schema({
-            self.p_root.get_child(ns.bse.foo, self.n_img, self.l_string, True)}))) # inconsistent w.r.t. node
+            self.p_root.child(ns.bse.foo, self.n_img, self.l_string, True)}))) # inconsistent w.r.t. node
         self.assertFalse(operator.ge(Schema({p_foo}), Schema({
-            self.p_root.get_child(ns.bse.foo, self.n_ent, self.l_string, False)}))) # inconsistent w.r.t. unique
+            self.p_root.child(ns.bse.foo, self.n_ent, self.l_string, False)}))) # inconsistent w.r.t. unique
         self.assertFalse(operator.ge(Schema({}, {self.n_img}), Schema({}, {
             types.Node(ns.bsfs.Image, types.Node(ns.bsfs.Node, None))})))
         self.assertFalse(operator.ge(Schema({}, {}, {self.l_integer}), Schema({}, {}, {
             types.Literal(ns.xsd.integer, types.Literal(ns.xsd.number, types.Literal(ns.bsfs.Literal, None)))})))
         # inconsistent schema cannot be a true superset
         self.assertFalse(operator.gt(Schema({p_foo}), Schema({
-            self.p_root.get_child(ns.bse.foo, self.n_ent, self.l_integer, True)}))) # inconsistent w.r.t. literal
+            self.p_root.child(ns.bse.foo, self.n_ent, self.l_integer, True)}))) # inconsistent w.r.t. literal
         self.assertFalse(operator.gt(Schema({p_foo}), Schema({
-            self.p_root.get_child(ns.bse.foo, self.n_img, self.l_string, True)}))) # inconsistent w.r.t. node
+            self.p_root.child(ns.bse.foo, self.n_img, self.l_string, True)}))) # inconsistent w.r.t. node
         self.assertFalse(operator.gt(Schema({p_foo}), Schema({
-            self.p_root.get_child(ns.bse.foo, self.n_ent, self.l_string, False)}))) # inconsistent w.r.t. unique
+            self.p_root.child(ns.bse.foo, self.n_ent, self.l_string, False)}))) # inconsistent w.r.t. unique
         self.assertFalse(operator.gt(Schema({}, {self.n_img}), Schema({}, {
             types.Node(ns.bsfs.Image, types.Node(ns.bsfs.Node, None))})))
         self.assertFalse(operator.gt(Schema({}, {}, {self.l_integer}), Schema({}, {}, {
@@ -351,26 +368,26 @@ class TestSchema(unittest.TestCase):
         # difference does not contain predicates from the RHS
         diff = Schema({self.p_tag, self.p_comment}).diff(Schema({self.p_group}))
         self.assertSetEqual(set(diff.nodes), set())
-        self.assertSetEqual(set(diff.literals), {self.l_root, self.l_string})
+        self.assertSetEqual(set(diff.literals), {self.l_string})
         self.assertSetEqual(set(diff.predicates), {self.p_comment})
 
         # difference considers extra nodes and literals
         diff = Schema({self.p_tag}, {self.n_unused}, {self.l_unused}).diff(Schema({self.p_tag}))
         self.assertSetEqual(set(diff.nodes), {self.n_unused})
-        self.assertSetEqual(set(diff.literals), {self.l_root, self.l_unused})
+        self.assertSetEqual(set(diff.literals), {self.l_unused})
         self.assertSetEqual(set(diff.predicates), set())
 
         # difference considers inconsistent types
         diff = Schema({self.p_tag}, {self.n_unused}, {self.l_unused}).diff(
             Schema({self.p_tag}, {types.Node(ns.bsfs.Unused, None)}, {types.Literal(ns.xsd.boolean, None)}))
         self.assertSetEqual(set(diff.nodes), {self.n_unused})
-        self.assertSetEqual(set(diff.literals), {self.l_root, self.l_unused})
+        self.assertSetEqual(set(diff.literals), {self.l_unused})
         self.assertSetEqual(set(diff.predicates), set())
 
         # __sub__ is an alias for diff
         diff = Schema({self.p_comment}, {self.n_unused}, {self.l_unused}) - Schema({self.p_group})
         self.assertSetEqual(set(diff.nodes), {self.n_unused})
-        self.assertSetEqual(set(diff.literals), {self.l_root, self.l_string, self.l_unused})
+        self.assertSetEqual(set(diff.literals), {self.l_string, self.l_unused})
         self.assertSetEqual(set(diff.predicates), {self.p_comment})
         # __sub__ only accepts Schema instances
         class Foo(): pass
@@ -547,196 +564,6 @@ class TestSchema(unittest.TestCase):
         self.assertFalse(schema.has_predicate(ns.bse.mimetype))
         self.assertFalse(schema.has_predicate(self.p_root))
 
-    def test_empty(self):
-        self.assertEqual(Schema.Empty(), Schema(
-            [types.Predicate(ns.bsfs.Predicate, None, types.Node(ns.bsfs.Node, None), None, False)],
-            [types.Node(ns.bsfs.Node, None)],
-            [types.Literal(ns.bsfs.Literal, None)],
-            ))
-
-    def test_from_string(self):
-        # from_string creates a schema
-        self.assertEqual(
-            Schema(self.predicates, self.nodes, self.literals),
-            Schema.from_string(self.schema_str))
-
-        # schema contains at least the root types
-        self.assertEqual(Schema.from_string(''), Schema({self.p_root}, {self.n_root}, {self.l_root}))
-
-        # custom example
-        self.assertEqual(
-            Schema({types.Predicate(ns.bsfs.Predicate, None, self.n_root, None, False).get_child(
-                ns.bse.filename, self.n_ent, self.l_string, False)}),
-            Schema.from_string('''
-                prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
-                prefix xsd: <http://www.w3.org/2001/XMLSchema#>
-                prefix bsfs: <http://bsfs.ai/schema/>
-                prefix bse: <http://bsfs.ai/schema/Entity#>
-
-                bsfs:Entity rdfs:subClassOf bsfs:Node .
-                xsd:string rdfs:subClassOf bsfs:Literal .
-
-                bse:filename rdfs:subClassOf bsfs:Predicate ;
-                    rdfs:domain bsfs:Entity ;
-                    rdfs:range xsd:string ;
-                    bsfs:unique "false"^^xsd:boolean .
-                '''))
-
-        # all nodes must be defined
-        self.assertRaises(errors.ConsistencyError, Schema.from_string, '''
-            prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
-            prefix xsd: <http://www.w3.org/2001/XMLSchema#>
-            prefix bsfs: <http://bsfs.ai/schema/>
-            prefix bse: <http://bsfs.ai/schema/Entity#>
-
-            xsd:string rdfs:subClassOf bsfs:Literal .
-
-            bse:filename rdfs:subClassOf bsfs:Predicate ;
-                rdfs:domain bsfs:Entity ;
-                rdfs:range xsd:string ;
-                bsfs:unique "false"^^xsd:boolean .
-            ''')
-
-        # all literals must be defined
-        self.assertRaises(errors.ConsistencyError, Schema.from_string, '''
-            prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
-            prefix xsd: <http://www.w3.org/2001/XMLSchema#>
-            prefix bsfs: <http://bsfs.ai/schema/>
-            prefix bse: <http://bsfs.ai/schema/Entity#>
-
-            bsfs:Entity rdfs:subClassOf bsfs:Node .
-
-            bse:filename rdfs:subClassOf bsfs:Predicate ;
-                rdfs:domain bsfs:Entity ;
-                rdfs:range xsd:string ;
-                bsfs:unique "false"^^xsd:boolean .
-            ''')
-
-        # must not have circular dependencies
-        self.assertRaises(errors.ConsistencyError, Schema.from_string, '''
-            prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
-            prefix bsfs: <http://bsfs.ai/schema/>
-            bsfs:Entity rdfs:subClassOf bsfs:Node .
-            # ah, a nice circular dependency
-            bsfs:Entity rdfs:subClassOf bsfs:Document .
-            bsfs:Document rdfs:subClassOf bsfs:Entity .
-            bsfs:PDF rdfs:subClassOf bsfs:Document .
-            ''')
-
-        # range must be a node or literal
-        self.assertRaises(errors.ConsistencyError, Schema.from_string, '''
-            prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
-            prefix xsd: <http://www.w3.org/2001/XMLSchema#>
-            prefix bsfs: <http://bsfs.ai/schema/>
-            prefix bse: <http://bsfs.ai/schema/Entity#>
-
-            bsfs:Entity rdfs:subClassOf bsfs:Node .
-
-            bse:filename rdfs:subClassOf bsfs:Predicate ;
-                rdfs:domain bsfs:Entity ;
-                rdfs:range xsd:string ;
-                bsfs:unique "false"^^xsd:boolean .
-            ''')
-        self.assertRaises(errors.ConsistencyError, Schema.from_string, '''
-            prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
-            prefix xsd: <http://www.w3.org/2001/XMLSchema#>
-            prefix bsfs: <http://bsfs.ai/schema/>
-            prefix bse: <http://bsfs.ai/schema/Entity#>
-
-            bsfs:Entity rdfs:subClassOf bsfs:Node .
-
-            bse:filename rdfs:subClassOf bsfs:Predicate ;
-                rdfs:domain bsfs:Entity ;
-                rdfs:range bsfs:Foo ;
-                bsfs:unique "false"^^xsd:boolean .
-            ''')
-        self.assertRaises(errors.ConsistencyError, Schema.from_string, '''
-            prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
-            prefix xsd: <http://www.w3.org/2001/XMLSchema#>
-            prefix bsfs: <http://bsfs.ai/schema/>
-            prefix bse: <http://bsfs.ai/schema/Entity#>
-
-            bsfs:Entity rdfs:subClassOf bsfs:Node .
-
-            bse:filename rdfs:subClassOf bsfs:Predicate ;
-                rdfs:domain bsfs:Entity ;
-                rdfs:range bsfs:Predicate ;
-                bsfs:unique "false"^^xsd:boolean .
-            ''')
-
-        # must be consistent
-        self.assertRaises(errors.ConsistencyError, Schema.from_string, '''
-            prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
-            prefix xsd: <http://www.w3.org/2001/XMLSchema#>
-            prefix bsfs: <http://bsfs.ai/schema/>
-
-            bsfs:Entity rdfs:subClassOf bsfs:Node .
-            bsfs:Document rdfs:subClassOf bsfs:Node .
-            bsfs:Document rdfs:subClassOf bsfs:Entity.
-            ''')
-        self.assertRaises(errors.ConsistencyError, Schema.from_string, '''
-            prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
-            prefix xsd: <http://www.w3.org/2001/XMLSchema#>
-            prefix bsfs: <http://bsfs.ai/schema/>
-
-            xsd:string rdfs:subClassOf bsfs:Literal .
-            xsd:name rdfs:subClassOf bsfs:Literal .
-            xsd:name rdfs:subClassOf xsd:string .
-            ''')
-        self.assertRaises(errors.ConsistencyError, Schema.from_string, '''
-            prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
-            prefix xsd: <http://www.w3.org/2001/XMLSchema#>
-            prefix bsfs: <http://bsfs.ai/schema/>
-            prefix bse: <http://bsfs.ai/schema/Entity#>
-
-            bsfs:Entity rdfs:subClassOf bsfs:Node .
-
-            bse:foo rdfs:subClassOf bsfs:Predicate ;
-                rdfs:domain bsfs:Node ;
-                rdfs:range bsfs:Node ;
-                bsfs:unique "false"^^xsd:boolean .
-
-            bse:foo rdfs:subClassOf bsfs:Predicate ;
-                rdfs:domain bsfs:Entity .
-
-            ''')
-        self.assertRaises(errors.ConsistencyError, Schema.from_string, '''
-            prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
-            prefix xsd: <http://www.w3.org/2001/XMLSchema#>
-            prefix bsfs: <http://bsfs.ai/schema/>
-            prefix bse: <http://bsfs.ai/schema/Entity#>
-
-            bsfs:Entity rdfs:subClassOf bsfs:Node .
-
-            bse:foo rdfs:subClassOf bsfs:Predicate ;
-                rdfs:domain bsfs:Node ;
-                rdfs:range bsfs:Node ;
-                bsfs:unique "false"^^xsd:boolean .
-
-            bse:foo rdfs:subClassOf bsfs:Predicate ;
-                rdfs:range bsfs:Entity .
-
-            ''')
-        self.assertRaises(errors.ConsistencyError, Schema.from_string, '''
-            prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
-            prefix xsd: <http://www.w3.org/2001/XMLSchema#>
-            prefix bsfs: <http://bsfs.ai/schema/>
-            prefix bse: <http://bsfs.ai/schema/Entity#>
-
-            bsfs:Entity rdfs:subClassOf bsfs:Node .
-
-            bse:foo rdfs:subClassOf bsfs:Predicate ;
-                rdfs:domain bsfs:Node ;
-                rdfs:range bsfs:Node ;
-                bsfs:unique "false"^^xsd:boolean .
-
-            bse:foo rdfs:subClassOf bsfs:Predicate ;
-                bsfs:unique "true"^^xsd:boolean .
-
-            ''')
-
-
-
 ## main ##
 
 if __name__ == '__main__':
diff --git a/test/schema/test_serialize.py b/test/schema/test_serialize.py
new file mode 100644
index 0000000..7d5d3ae
--- /dev/null
+++ b/test/schema/test_serialize.py
@@ -0,0 +1,1048 @@
+
+# imports
+import re
+import unittest
+
+# bsfs imports
+from bsfs.namespace import ns
+from bsfs.schema import Schema, types
+from bsfs.utils import errors, URI
+
+# objects to test
+from bsfs.schema.serialize import from_string, to_string
+
+
+## code ##
+
+ns.bse = ns.bsfs.Entity()
+
+class TestFromString(unittest.TestCase):
+
+    def test_empty(self):
+        # schema contains at least the root types
+        self.assertEqual(from_string(''), Schema())
+
+
+    def test_circular_dependency(self):
+        # must not have circular dependencies
+        self.assertRaises(errors.ConsistencyError, from_string, '''
+            prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
+            prefix bsfs: <https://schema.bsfs.io/core/>
+            bsfs:Entity rdfs:subClassOf bsfs:Node .
+            # ah, a nice circular dependency
+            bsfs:Entity rdfs:subClassOf bsfs:Document .
+            bsfs:Document rdfs:subClassOf bsfs:Entity .
+            bsfs:PDF rdfs:subClassOf bsfs:Document .
+            ''')
+
+
+    def test_node(self):
+        # all nodes must be defined
+        self.assertRaises(errors.ConsistencyError, from_string, '''
+            prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
+            prefix xsd: <http://www.w3.org/2001/XMLSchema#>
+            prefix bsfs: <https://schema.bsfs.io/core/>
+            prefix bse: <https://schema.bsfs.io/core/Entity#>
+
+            xsd:string rdfs:subClassOf bsfs:Literal .
+
+            bse:filename rdfs:subClassOf bsfs:Predicate ;
+                rdfs:domain bsfs:Entity ;
+                rdfs:range xsd:string ;
+                bsfs:unique "false"^^xsd:boolean .
+            ''')
+
+        # node definitions must be consistent (cannot re-use a node uri)
+        self.assertRaises(errors.ConsistencyError, from_string, '''
+            prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
+            prefix xsd: <http://www.w3.org/2001/XMLSchema#>
+            prefix bsfs: <https://schema.bsfs.io/core/>
+
+            bsfs:Entity rdfs:subClassOf bsfs:Node .
+            bsfs:Document rdfs:subClassOf bsfs:Node .
+            bsfs:Document rdfs:subClassOf bsfs:Entity . # conflicting parent
+            ''')
+
+        # additional nodes can be defined
+        n_unused = types.ROOT_NODE.child(ns.bsfs.unused)
+        self.assertEqual(Schema({}, {n_unused}), from_string('''
+            prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
+            prefix xsd: <http://www.w3.org/2001/XMLSchema#>
+            prefix bsfs: <https://schema.bsfs.io/core/>
+            prefix bse: <https://schema.bsfs.io/core/Entity#>
+
+            bsfs:unused rdfs:subClassOf bsfs:Node . # unused symbol
+            '''))
+
+        # a node can have multiple children
+        n_ent = types.ROOT_NODE.child(ns.bsfs.Entity)
+        n_tag = types.ROOT_NODE.child(ns.bsfs.Tag)
+        n_doc = n_ent.child(ns.bsfs.Document)
+        n_image = n_ent.child(ns.bsfs.Image)
+        self.assertEqual(Schema({}, {n_ent, n_tag, n_doc, n_image}), from_string('''
+            prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
+            prefix xsd: <http://www.w3.org/2001/XMLSchema#>
+            prefix bsfs: <https://schema.bsfs.io/core/>
+            prefix bse: <https://schema.bsfs.io/core/Entity#>
+
+            # nodes inherit from same parent
+            bsfs:Entity rdfs:subClassOf bsfs:Node .
+            bsfs:Tag rdfs:subClassOf bsfs:Node .
+
+            # nodes inherit from same parent
+            bsfs:Document rdfs:subClassOf bsfs:Entity .
+            bsfs:Image rdfs:subClassOf bsfs:Entity .
+            '''))
+
+        # additional nodes can be defined and used
+        n_ent = types.ROOT_NODE.child(ns.bsfs.Entity)
+        l_string = types.ROOT_LITERAL.child(ns.xsd.string)
+        p_filename = types.ROOT_PREDICATE.child(ns.bse.filename,
+            n_ent, l_string, False)
+        self.assertEqual(Schema({p_filename}), from_string('''
+            prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
+            prefix xsd: <http://www.w3.org/2001/XMLSchema#>
+            prefix bsfs: <https://schema.bsfs.io/core/>
+            prefix bse: <https://schema.bsfs.io/core/Entity#>
+
+            bsfs:Entity rdfs:subClassOf bsfs:Node .
+            xsd:string rdfs:subClassOf bsfs:Literal .
+
+            bse:filename rdfs:subClassOf bsfs:Predicate ;
+                rdfs:domain bsfs:Entity ;
+                rdfs:range xsd:string ;
+                bsfs:unique "false"^^xsd:boolean .
+            '''))
+
+        # nodes can have annotations
+        self.assertDictEqual(from_string('''
+            prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
+            prefix xsd: <http://www.w3.org/2001/XMLSchema#>
+            prefix bsfs: <https://schema.bsfs.io/core/>
+
+            bsfs:Entity rdfs:subClassOf bsfs:Node .
+
+            ''').node(ns.bsfs.Entity).annotations, {})
+        self.assertDictEqual(from_string('''
+            prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
+            prefix xsd: <http://www.w3.org/2001/XMLSchema#>
+            prefix bsfs: <https://schema.bsfs.io/core/>
+
+            bsfs:Entity rdfs:subClassOf bsfs:Node ;
+                rdfs:label "hello world"^^xsd:string ;
+                bsfs:foo "1234"^^xsd:integer .
+
+            ''').node(ns.bsfs.Entity).annotations, {
+            ns.rdfs.label: 'hello world',
+            ns.bsfs.foo: 1234,
+            })
+
+
+    def test_literal(self):
+        # all literals must be defined
+        self.assertRaises(errors.ConsistencyError, from_string, '''
+            prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
+            prefix xsd: <http://www.w3.org/2001/XMLSchema#>
+            prefix bsfs: <https://schema.bsfs.io/core/>
+            prefix bse: <https://schema.bsfs.io/core/Entity#>
+
+            bsfs:Entity rdfs:subClassOf bsfs:Node .
+
+            bse:filename rdfs:subClassOf bsfs:Predicate ;
+                rdfs:domain bsfs:Entity ;
+                rdfs:range xsd:string ; # undefined symbol
+                bsfs:unique "false"^^xsd:boolean .
+            ''')
+
+        # literal definitions must be consistent (cannot re-use a literal uri)
+        self.assertRaises(errors.ConsistencyError, from_string, '''
+            prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
+            prefix xsd: <http://www.w3.org/2001/XMLSchema#>
+            prefix bsfs: <https://schema.bsfs.io/core/>
+
+            xsd:string rdfs:subClassOf bsfs:Literal .
+            xsd:name rdfs:subClassOf bsfs:Literal .
+            xsd:name rdfs:subClassOf xsd:string . # conflicting parent
+            ''')
+
+        # additional literals can be defined
+        l_unused = types.ROOT_LITERAL.child(ns.xsd.unused)
+        self.assertEqual(Schema({}, {}, {l_unused}), from_string('''
+            prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
+            prefix xsd: <http://www.w3.org/2001/XMLSchema#>
+            prefix bsfs: <https://schema.bsfs.io/core/>
+            prefix bse: <https://schema.bsfs.io/core/Entity#>
+
+            xsd:unused rdfs:subClassOf bsfs:Literal . # unused symbol
+            '''))
+
+        # a literal can have multiple children
+        l_string = types.ROOT_LITERAL.child(ns.xsd.string)
+        l_integer = types.ROOT_NUMBER.child(ns.xsd.integer)
+        l_unsigned = l_integer.child(ns.xsd.unsigned)
+        l_signed = l_integer.child(ns.xsd.signed)
+        self.assertEqual(Schema({}, {}, {l_string, l_integer, l_unsigned, l_signed}), from_string('''
+            prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
+            prefix xsd: <http://www.w3.org/2001/XMLSchema#>
+            prefix bsfs: <https://schema.bsfs.io/core/>
+            prefix bse: <https://schema.bsfs.io/core/Entity#>
+            prefix bsl: <https://schema.bsfs.io/core/Literal/>
+
+            # literals inherit from same parent
+            xsd:string rdfs:subClassOf bsfs:Literal .
+            bsl:Number rdfs:subClassOf bsfs:Literal .
+            xsd:integer rdfs:subClassOf bsl:Number .
+
+            # literals inherit from same parent
+            xsd:unsigned rdfs:subClassOf xsd:integer .
+            xsd:signed rdfs:subClassOf xsd:integer .
+            '''))
+
+        # additional literals can be defined and used
+        n_ent = types.ROOT_NODE.child(ns.bsfs.Entity)
+        l_string = types.ROOT_LITERAL.child(ns.xsd.string)
+        p_filename = types.ROOT_PREDICATE.child(ns.bse.filename,
+            n_ent, l_string, False)
+        self.assertEqual(Schema({p_filename}), from_string('''
+            prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
+            prefix xsd: <http://www.w3.org/2001/XMLSchema#>
+            prefix bsfs: <https://schema.bsfs.io/core/>
+            prefix bse: <https://schema.bsfs.io/core/Entity#>
+
+            bsfs:Entity rdfs:subClassOf bsfs:Node .
+            xsd:string rdfs:subClassOf bsfs:Literal .
+
+            bse:filename rdfs:subClassOf bsfs:Predicate ;
+                rdfs:domain bsfs:Entity ;
+                rdfs:range xsd:string ;
+                bsfs:unique "false"^^xsd:boolean .
+            '''))
+
+        # literals can have annotations
+        self.assertDictEqual(from_string('''
+            prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
+            prefix xsd: <http://www.w3.org/2001/XMLSchema#>
+            prefix bsfs: <https://schema.bsfs.io/core/>
+
+            xsd:string rdfs:subClassOf bsfs:Literal .
+
+            ''').literal(ns.xsd.string).annotations, {})
+        self.assertDictEqual(from_string('''
+            prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
+            prefix xsd: <http://www.w3.org/2001/XMLSchema#>
+            prefix bsfs: <https://schema.bsfs.io/core/>
+
+            xsd:string rdfs:subClassOf bsfs:Literal ;
+                rdfs:label "hello world"^^xsd:string ;
+                bsfs:foo "1234"^^xsd:integer .
+
+            ''').literal(ns.xsd.string).annotations, {
+            ns.rdfs.label: 'hello world',
+            ns.bsfs.foo: 1234,
+            })
+
+
+    def test_predicate(self):
+        # domain must be defined
+        self.assertRaises(errors.ConsistencyError, from_string, '''
+            prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
+            prefix xsd: <http://www.w3.org/2001/XMLSchema#>
+            prefix bsfs: <https://schema.bsfs.io/core/>
+            prefix bse: <https://schema.bsfs.io/core/Entity#>
+
+            xsd:string rdfs:subClassOf bsfs:Literal .
+
+            bse:filename rdfs:subClassOf bsfs:Predicate ;
+                rdfs:domain bsfs:Entity ; # undefined symbol
+                rdfs:range xsd:string ;
+                bsfs:unique "false"^^xsd:boolean .
+            ''')
+        # domain cannot be a literal
+        self.assertRaises(errors.ConsistencyError, from_string, '''
+            prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
+            prefix xsd: <http://www.w3.org/2001/XMLSchema#>
+            prefix bsfs: <https://schema.bsfs.io/core/>
+            prefix bse: <https://schema.bsfs.io/core/Entity#>
+
+            bsfs:Entity rdfs:subClassOf bsfs:Literal .
+            xsd:string rdfs:subClassOf bsfs:Literal .
+
+            bse:filename rdfs:subClassOf bsfs:Predicate ;
+                rdfs:domain bsfs:Entity ; # literal instead of node
+                rdfs:range xsd:string ;
+                bsfs:unique "false"^^xsd:boolean .
+            ''')
+
+        # range must be defined
+        self.assertRaises(errors.ConsistencyError, from_string, '''
+            prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
+            prefix xsd: <http://www.w3.org/2001/XMLSchema#>
+            prefix bsfs: <https://schema.bsfs.io/core/>
+            prefix bse: <https://schema.bsfs.io/core/Entity#>
+
+            bsfs:Entity rdfs:subClassOf bsfs:Node .
+
+            bse:filename rdfs:subClassOf bsfs:Predicate ;
+                rdfs:domain bsfs:Entity ;
+                rdfs:range xsd:string ; # undefined symbol
+                bsfs:unique "false"^^xsd:boolean .
+            ''')
+        # range must be defined
+        self.assertRaises(errors.ConsistencyError, from_string, '''
+            prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
+            prefix xsd: <http://www.w3.org/2001/XMLSchema#>
+            prefix bsfs: <https://schema.bsfs.io/core/>
+            prefix bse: <https://schema.bsfs.io/core/Entity#>
+
+            bsfs:Entity rdfs:subClassOf bsfs:Node .
+
+            bse:filename rdfs:subClassOf bsfs:Predicate ;
+                rdfs:domain bsfs:Entity ;
+                rdfs:range bsfs:Foo ; # undefined symbol
+                bsfs:unique "false"^^xsd:boolean .
+            ''')
+        # range must be a node or a literal
+        self.assertRaises(errors.ConsistencyError, from_string, '''
+            prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
+            prefix xsd: <http://www.w3.org/2001/XMLSchema#>
+            prefix bsfs: <https://schema.bsfs.io/core/>
+            prefix bse: <https://schema.bsfs.io/core/Entity#>
+
+            bsfs:Entity rdfs:subClassOf bsfs:Node .
+
+            bse:filename rdfs:subClassOf bsfs:Predicate ;
+                rdfs:domain bsfs:Entity ;
+                rdfs:range bsfs:Predicate ; # invalid symbol
+                bsfs:unique "false"^^xsd:boolean .
+            ''')
+
+        # additional predicates can be defined
+        n_ent = types.ROOT_NODE.child(ns.bsfs.Entity)
+        l_string = types.ROOT_LITERAL.child(ns.xsd.string)
+        p_comment = types.ROOT_PREDICATE.child(ns.bse.comment, domain=n_ent, range=l_string, unique=False)
+        self.assertEqual(Schema({p_comment}), from_string('''
+            prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
+            prefix xsd: <http://www.w3.org/2001/XMLSchema#>
+            prefix bsfs: <https://schema.bsfs.io/core/>
+            prefix bse: <https://schema.bsfs.io/core/Entity#>
+
+            bsfs:Entity rdfs:subClassOf bsfs:Node .
+            xsd:string rdfs:subClassOf bsfs:Literal .
+
+            bse:comment rdfs:subClassOf bsfs:Predicate ;
+                rdfs:domain bsfs:Entity ;
+                rdfs:range xsd:string ;
+                bsfs:unique "false"^^xsd:boolean .
+            '''))
+
+        # predicates inherit properties from parents
+        n_ent = types.ROOT_NODE.child(ns.bsfs.Entity)
+        l_string = types.ROOT_LITERAL.child(ns.xsd.string)
+        p_annotation = types.ROOT_PREDICATE.child(ns.bsfs.Annotation, domain=n_ent, range=l_string)
+        p_comment = p_annotation.child(ns.bse.comment, unique=True)
+        self.assertEqual(Schema({p_comment}), from_string('''
+            prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
+            prefix xsd: <http://www.w3.org/2001/XMLSchema#>
+            prefix bsfs: <https://schema.bsfs.io/core/>
+            prefix bse: <https://schema.bsfs.io/core/Entity#>
+
+            bsfs:Entity rdfs:subClassOf bsfs:Node .
+            xsd:string rdfs:subClassOf bsfs:Literal .
+
+            bsfs:Annotation rdfs:subClassOf bsfs:Predicate ;
+                rdfs:domain bsfs:Entity ;
+                rdfs:range xsd:string .
+
+            bse:comment rdfs:subClassOf bsfs:Annotation ; # inherits domain/range from bsfs:Annotation
+                bsfs:unique "true"^^xsd:boolean .
+            '''))
+
+        # we can define partial predicates (w/o specifying a usable range)
+        n_ent = types.ROOT_NODE.child(ns.bsfs.Entity)
+        l_string = types.ROOT_LITERAL.child(ns.xsd.string)
+        p_annotation = types.ROOT_PREDICATE.child(ns.bsfs.Annotation, domain=n_ent)
+        p_comment = p_annotation.child(ns.bse.comment, range=l_string, unique=False)
+        self.assertEqual(Schema({p_comment}), from_string('''
+            prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
+            prefix xsd: <http://www.w3.org/2001/XMLSchema#>
+            prefix bsfs: <https://schema.bsfs.io/core/>
+            prefix bse: <https://schema.bsfs.io/core/Entity#>
+
+            bsfs:Entity rdfs:subClassOf bsfs:Node .
+            xsd:string rdfs:subClassOf bsfs:Literal .
+
+            bsfs:Annotation rdfs:subClassOf bsfs:Predicate ; # derive predicate w/o setting range
+                rdfs:domain bsfs:Entity .
+
+            bse:comment rdfs:subClassOf bsfs:Annotation ; # derived predicate w/ setting range
+                rdfs:range xsd:string ;
+                bsfs:unique "false"^^xsd:boolean .
+            '''))
+
+        # predicate definition can be split across multiple statements.
+        # statements can be repeated
+        n_ent = types.ROOT_NODE.child(ns.bsfs.Entity)
+        p_foo = types.ROOT_PREDICATE.child(ns.bse.foo, domain=n_ent, range=types.ROOT_NODE, unique=True)
+        self.assertEqual(Schema({p_foo}), from_string('''
+            prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
+            prefix xsd: <http://www.w3.org/2001/XMLSchema#>
+            prefix bsfs: <https://schema.bsfs.io/core/>
+            prefix bse: <https://schema.bsfs.io/core/Entity#>
+
+            bsfs:Entity rdfs:subClassOf bsfs:Node .
+
+            bse:foo rdfs:subClassOf bsfs:Predicate ;
+                rdfs:range bsfs:Node ;
+                bsfs:unique "true"^^xsd:boolean .
+
+            bse:foo rdfs:subClassOf bsfs:Predicate ;
+                rdfs:domain bsfs:Entity.
+            '''))
+
+        # domain must be a subtype of parent's domain
+        n_ent = types.ROOT_NODE.child(ns.bsfs.Entity)
+        n_image = n_ent.child(ns.bsfs.Image)
+        p_foo = types.ROOT_PREDICATE.child(ns.bse.foo, domain=types.ROOT_NODE)
+        p_bar = p_foo.child(ns.bse.bar, domain=n_ent)
+        p_foobar = p_bar.child(ns.bse.foobar, domain=n_image)
+        self.assertEqual(Schema({p_foobar}), from_string('''
+            prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
+            prefix xsd: <http://www.w3.org/2001/XMLSchema#>
+            prefix bsfs: <https://schema.bsfs.io/core/>
+            prefix bse: <https://schema.bsfs.io/core/Entity#>
+
+            bsfs:Entity rdfs:subClassOf bsfs:Node .
+            bsfs:Image rdfs:subClassOf bsfs:Entity .
+
+            bse:foo rdfs:subClassOf bsfs:Predicate ;
+                rdfs:domain bsfs:Node .
+            bse:bar rdfs:subClassOf bse:foo ;
+                rdfs:domain bsfs:Entity .
+            bse:foobar rdfs:subClassOf bse:bar ;
+                rdfs:domain bsfs:Image .
+            '''))
+        self.assertRaises(errors.ConsistencyError, from_string, '''
+            prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
+            prefix xsd: <http://www.w3.org/2001/XMLSchema#>
+            prefix bsfs: <https://schema.bsfs.io/core/>
+            prefix bse: <https://schema.bsfs.io/core/Entity#>
+
+            bsfs:Entity rdfs:subClassOf bsfs:Node .
+            bsfs:Image rdfs:subClassOf bsfs:Entity .
+
+            bse:foo rdfs:subClassOf bsfs:Predicate ;
+                rdfs:domain bsfs:Image .
+            bse:bar rdfs:subClassOf bse:foo ;
+                rdfs:domain bsfs:Entity .
+            bse:foobar rdfs:subClassOf bse:bar ;
+                rdfs:domain bsfs:Node .
+            ''')
+
+        # range must be a subtype of parent's range
+        n_ent = types.ROOT_NODE.child(ns.bsfs.Entity)
+        n_image = n_ent.child(ns.bsfs.Image)
+        p_foo = types.ROOT_PREDICATE.child(ns.bse.foo, range=types.ROOT_NODE)
+        p_bar = p_foo.child(ns.bse.bar, range=n_ent)
+        p_foobar = p_bar.child(ns.bse.foobar, range=n_image)
+        self.assertEqual(Schema({p_foobar}), from_string('''
+            prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
+            prefix xsd: <http://www.w3.org/2001/XMLSchema#>
+            prefix bsfs: <https://schema.bsfs.io/core/>
+            prefix bse: <https://schema.bsfs.io/core/Entity#>
+
+            bsfs:Entity rdfs:subClassOf bsfs:Node .
+            bsfs:Image rdfs:subClassOf bsfs:Entity .
+
+            bse:foo rdfs:subClassOf bsfs:Predicate ;
+                rdfs:range bsfs:Node .
+            bse:bar rdfs:subClassOf bse:foo ;
+                rdfs:range bsfs:Entity .
+            bse:foobar rdfs:subClassOf bse:bar ;
+                rdfs:range bsfs:Image .
+            '''))
+        self.assertRaises(errors.ConsistencyError, from_string, '''
+            prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
+            prefix xsd: <http://www.w3.org/2001/XMLSchema#>
+            prefix bsfs: <https://schema.bsfs.io/core/>
+            prefix bse: <https://schema.bsfs.io/core/Entity#>
+
+            bsfs:Entity rdfs:subClassOf bsfs:Node .
+            bsfs:Image rdfs:subClassOf bsfs:Entity .
+
+            bse:foo rdfs:subClassOf bsfs:Predicate ;
+                rdfs:range bsfs:Image .
+            bse:bar rdfs:subClassOf bse:foo ;
+                rdfs:range bsfs:Entity .
+            bse:foobar rdfs:subClassOf bse:bar ;
+                rdfs:range bsfs:Node .
+            ''')
+
+        # cannot define the same predicate from multiple parents
+        self.assertRaises(errors.ConsistencyError, from_string, '''
+            prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
+            prefix xsd: <http://www.w3.org/2001/XMLSchema#>
+            prefix bsfs: <https://schema.bsfs.io/core/>
+            prefix bse: <https://schema.bsfs.io/core/Entity#>
+
+            bsfs:Entity rdfs:subClassOf bsfs:Node .
+            bsfs:Annotation rdfs:subClassOf bsfs:Predicate .
+
+            bse:foo rdfs:subClassOf bsfs:Predicate ;
+                rdfs:domain bsfs:Node ;
+                rdfs:range bsfs:Node ;
+                bsfs:unique "false"^^xsd:boolean .
+
+            bse:foo rdfs:subClassOf bsfs:Annotation ;
+                rdfs:domain bsfs:Node ;
+                rdfs:range bsfs:Node ;
+                bsfs:unique "false"^^xsd:boolean .
+
+            ''')
+        # cannot assign multiple conflicting domains to the same predicate
+        self.assertRaises(errors.ConsistencyError, from_string, '''
+            prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
+            prefix xsd: <http://www.w3.org/2001/XMLSchema#>
+            prefix bsfs: <https://schema.bsfs.io/core/>
+            prefix bse: <https://schema.bsfs.io/core/Entity#>
+
+            bsfs:Entity rdfs:subClassOf bsfs:Node .
+
+            bse:foo rdfs:subClassOf bsfs:Predicate ;
+                rdfs:domain bsfs:Node ;
+                rdfs:range bsfs:Node ;
+                bsfs:unique "false"^^xsd:boolean .
+
+            bse:foo rdfs:subClassOf bsfs:Predicate ;
+                rdfs:domain bsfs:Entity . # conflicting domain
+            ''')
+        # cannot assign multiple conflicting ranges to the same predicate
+        self.assertRaises(errors.ConsistencyError, from_string, '''
+            prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
+            prefix xsd: <http://www.w3.org/2001/XMLSchema#>
+            prefix bsfs: <https://schema.bsfs.io/core/>
+            prefix bse: <https://schema.bsfs.io/core/Entity#>
+
+            bsfs:Entity rdfs:subClassOf bsfs:Node .
+
+            bse:foo rdfs:subClassOf bsfs:Predicate ;
+                rdfs:domain bsfs:Node ;
+                rdfs:range bsfs:Node ;
+                bsfs:unique "false"^^xsd:boolean .
+
+            bse:foo rdfs:subClassOf bsfs:Predicate ;
+                rdfs:range bsfs:Entity . # conflicting range
+            ''')
+        # cannot assign multiple conflicting uniques to the same predicate
+        self.assertRaises(errors.ConsistencyError, from_string, '''
+            prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
+            prefix xsd: <http://www.w3.org/2001/XMLSchema#>
+            prefix bsfs: <https://schema.bsfs.io/core/>
+            prefix bse: <https://schema.bsfs.io/core/Entity#>
+
+            bsfs:Entity rdfs:subClassOf bsfs:Node .
+
+            bse:foo rdfs:subClassOf bsfs:Predicate ;
+                rdfs:domain bsfs:Node ;
+                rdfs:range bsfs:Node ;
+                bsfs:unique "false"^^xsd:boolean .
+
+            bse:foo rdfs:subClassOf bsfs:Predicate ;
+                bsfs:unique "true"^^xsd:boolean . # conflicting unique
+            ''')
+
+        # predicates can have annotations
+        self.assertDictEqual(from_string('''
+            prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
+            prefix xsd: <http://www.w3.org/2001/XMLSchema#>
+            prefix bsfs: <https://schema.bsfs.io/core/>
+            prefix bse: <https://schema.bsfs.io/core/Entity#>
+
+            bse:comment rdfs:subClassOf bsfs:Predicate ;
+                rdfs:range bsfs:Node .
+
+            ''').predicate(ns.bse.comment).annotations, {})
+        self.assertDictEqual(from_string('''
+            prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
+            prefix xsd: <http://www.w3.org/2001/XMLSchema#>
+            prefix bsfs: <https://schema.bsfs.io/core/>
+            prefix bse: <https://schema.bsfs.io/core/Entity#>
+
+            bse:comment rdfs:subClassOf bsfs:Predicate ;
+                rdfs:range bsfs:Node ;
+                rdfs:label "hello world"^^xsd:string ;
+                bsfs:foo "1234"^^xsd:integer .
+
+            ''').predicate(ns.bse.comment).annotations, {
+            ns.rdfs.label: 'hello world',
+            ns.bsfs.foo: 1234,
+            })
+
+
+    def test_feature(self):
+        # additional features can be defined
+        f_colors = types.ROOT_FEATURE.child(ns.bsfs.Colors)
+        self.assertEqual(Schema(literals={f_colors}), from_string('''
+            prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
+            prefix xsd: <http://www.w3.org/2001/XMLSchema#>
+            prefix bsfs: <https://schema.bsfs.io/core/>
+            prefix bse: <https://schema.bsfs.io/core/Entity#>
+            prefix bsl: <https://schema.bsfs.io/core/Literal/>
+            prefix bsa: <https://schema.bsfs.io/core/Literal/Array/>
+
+            bsl:Array rdfs:subClassOf bsfs:Literal .
+            bsa:Feature rdfs:subClassOf bsl:Array.
+
+            bsfs:Colors rdfs:subClassOf bsa:Feature .
+
+            '''))
+
+        # features inherit properties from parents
+        f_colors = types.ROOT_FEATURE.child(ns.bsfs.Colors, dimension=1234, dtype=ns.bsfs.dtype().i32)
+        f_main_colors = f_colors.child(ns.bsfs.MainColor, distance=ns.bsfs.cosine, dtype=ns.bsfs.dtype().f16)
+        self.assertEqual(Schema(literals={f_colors, f_main_colors}), from_string('''
+            prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
+            prefix xsd: <http://www.w3.org/2001/XMLSchema#>
+            prefix bsfs: <https://schema.bsfs.io/core/>
+            prefix bse: <https://schema.bsfs.io/core/Entity#>
+            prefix bsl: <https://schema.bsfs.io/core/Literal/>
+            prefix bsa: <https://schema.bsfs.io/core/Literal/Array/>
+
+            bsl:Array rdfs:subClassOf bsfs:Literal .
+            bsa:Feature rdfs:subClassOf bsl:Array.
+
+            bsfs:Colors rdfs:subClassOf bsa:Feature ; # inherits distance from bsa:Feature
+                bsfs:dimension "1234"^^xsd:integer ; # overwrites bsa:Feature
+                bsfs:dtype <https://schema.bsfs.io/core/dtype#i32> . # overwrites bsa:Feature
+
+            bsfs:MainColor rdfs:subClassOf bsfs:Colors ; # inherits dimension from bsfs:Colors
+                bsfs:distance bsfs:cosine ; # overwrites bsa:Feature
+                bsfs:dtype <https://schema.bsfs.io/core/dtype#f16> . # overwrites bsfs:Colors
+
+            '''))
+
+        # feature definition can be split across multiple statements.
+        # statements can be repeated
+        f_colors = types.ROOT_FEATURE.child(ns.bsfs.Colors, dimension=1234, dtype=ns.bsfs.dtype().f32)
+        self.assertEqual(Schema(literals={f_colors}), from_string('''
+            prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
+            prefix xsd: <http://www.w3.org/2001/XMLSchema#>
+            prefix bsfs: <https://schema.bsfs.io/core/>
+            prefix bse: <https://schema.bsfs.io/core/Entity#>
+            prefix bsl: <https://schema.bsfs.io/core/Literal/>
+            prefix bsa: <https://schema.bsfs.io/core/Literal/Array/>
+
+            bsl:Array rdfs:subClassOf bsfs:Literal .
+            bsa:Feature rdfs:subClassOf bsl:Array.
+
+            bsfs:Colors rdfs:subClassOf bsa:Feature ;
+                bsfs:dimension "1234"^^xsd:integer .
+
+            bsfs:Colors rdfs:subClassOf bsa:Feature ;
+                bsfs:dimension "1234"^^xsd:integer ; # non-conflicting repetition
+                bsfs:dtype <https://schema.bsfs.io/core/dtype#f32> .
+            '''))
+
+        # cannot define the same feature from multiple parents
+        self.assertRaises(errors.ConsistencyError, from_string, '''
+            prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
+            prefix xsd: <http://www.w3.org/2001/XMLSchema#>
+            prefix bsfs: <https://schema.bsfs.io/core/>
+            prefix bse: <https://schema.bsfs.io/core/Entity#>
+            prefix bsl: <https://schema.bsfs.io/core/Literal/>
+            prefix bsa: <https://schema.bsfs.io/core/Literal/Array/>
+
+            bsl:Array rdfs:subClassOf bsfs:Literal .
+            bsa:Feature rdfs:subClassOf bsl:Array.
+            bsfs:ColorSpace rdfs:subClassOf bsa:Feature .
+
+            bsfs:Colors rdfs:subClassOf bsa:Feature .
+            bsfs:Colors rdfs:subClassOf bsfs:ColorSpace .
+
+            ''')
+        # cannot assign multiple conflicting dimensions to the same feature
+        self.assertRaises(errors.ConsistencyError, from_string, '''
+            prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
+            prefix xsd: <http://www.w3.org/2001/XMLSchema#>
+            prefix bsfs: <https://schema.bsfs.io/core/>
+            prefix bse: <https://schema.bsfs.io/core/Entity#>
+            prefix bsl: <https://schema.bsfs.io/core/Literal/>
+            prefix bsa: <https://schema.bsfs.io/core/Literal/Array/>
+
+            bsl:Array rdfs:subClassOf bsfs:Literal .
+            bsa:Feature rdfs:subClassOf bsl:Array.
+
+            bsfs:Colors rdfs:subClassOf bsa:Feature ;
+                bsfs:dimension "1234"^^xsd:integer .
+
+            bsfs:Colors rdfs:subClassOf bsa:Feature ;
+                bsfs:dimension "4321"^^xsd:integer . # conflicting dimension
+
+            ''')
+        # cannot assign multiple conflicting dtypes to the same feature
+        self.assertRaises(errors.ConsistencyError, from_string, '''
+            prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
+            prefix xsd: <http://www.w3.org/2001/XMLSchema#>
+            prefix bsfs: <https://schema.bsfs.io/core/>
+            prefix bse: <https://schema.bsfs.io/core/Entity#>
+            prefix bsl: <https://schema.bsfs.io/core/Literal/>
+            prefix bsa: <https://schema.bsfs.io/core/Literal/Array/>
+
+            bsl:Array rdfs:subClassOf bsfs:Literal .
+            bsa:Feature rdfs:subClassOf bsl:Array.
+
+            bsfs:Colors rdfs:subClassOf bsa:Feature ;
+                bsfs:dtype <https://schema.bsfs.io/core/dtype#i32> .
+
+            bsfs:Colors rdfs:subClassOf bsa:Feature ;
+                bsfs:dtype <https://schema.bsfs.io/core/dtype#f16> .  # conflicting dtype
+            ''')
+        # cannot assign multiple conflicting distance metrics to the same feature
+        self.assertRaises(errors.ConsistencyError, from_string, '''
+            prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
+            prefix xsd: <http://www.w3.org/2001/XMLSchema#>
+            prefix bsfs: <https://schema.bsfs.io/core/>
+            prefix bse: <https://schema.bsfs.io/core/Entity#>
+            prefix bsl: <https://schema.bsfs.io/core/Literal/>
+            prefix bsa: <https://schema.bsfs.io/core/Literal/Array/>
+
+            bsl:Array rdfs:subClassOf bsfs:Literal .
+            bsa:Feature rdfs:subClassOf bsl:Array.
+
+            bsfs:Colors rdfs:subClassOf bsa:Feature ;
+                bsfs:distance bsfs:euclidean .
+
+            bsfs:Colors rdfs:subClassOf bsa:Feature ;
+                bsfs:distance bsfs:cosine . # conflicting distance
+            ''')
+
+        # features can have annotations
+        self.assertDictEqual(from_string('''
+            prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
+            prefix xsd: <http://www.w3.org/2001/XMLSchema#>
+            prefix bsfs: <https://schema.bsfs.io/core/>
+            prefix bse: <https://schema.bsfs.io/core/Entity#>
+            prefix bsl: <https://schema.bsfs.io/core/Literal/>
+            prefix bsa: <https://schema.bsfs.io/core/Literal/Array/>
+
+            bsl:Array rdfs:subClassOf bsfs:Literal .
+            bsa:Feature rdfs:subClassOf bsl:Array.
+
+            bsfs:Colors rdfs:subClassOf bsa:Feature ;
+                bsfs:dimension "1234"^^xsd:integer .
+
+            ''').literal(ns.bsfs.Colors).annotations, {})
+        self.assertDictEqual(from_string('''
+            prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
+            prefix xsd: <http://www.w3.org/2001/XMLSchema#>
+            prefix bsfs: <https://schema.bsfs.io/core/>
+            prefix bse: <https://schema.bsfs.io/core/Entity#>
+            prefix bsl: <https://schema.bsfs.io/core/Literal/>
+            prefix bsa: <https://schema.bsfs.io/core/Literal/Array/>
+
+            bsl:Array rdfs:subClassOf bsfs:Literal .
+            bsa:Feature rdfs:subClassOf bsl:Array.
+
+            bsfs:Colors rdfs:subClassOf bsa:Feature ;
+                bsfs:dimension "1234"^^xsd:integer ;
+                rdfs:label "hello world"^^xsd:string ;
+                bsfs:foo "1234"^^xsd:integer .
+
+            ''').literal(ns.bsfs.Colors).annotations, {
+            ns.rdfs.label: 'hello world',
+            ns.bsfs.foo: 1234,
+            })
+
+
+    def test_integration(self):
+        # nodes
+        n_ent = types.ROOT_NODE.child(ns.bsfs.Entity)
+        n_tag = types.ROOT_NODE.child(ns.bsfs.Tag)
+        n_image = n_ent.child(ns.bsfs.Image)
+        # literals
+        l_string = types.ROOT_LITERAL.child(ns.xsd.string)
+        l_array = types.ROOT_LITERAL.child(ns.bsfs.array)
+        l_integer = types.ROOT_NUMBER.child(ns.xsd.integer)
+        l_boolean = types.ROOT_LITERAL.child(ns.xsd.boolean)
+        # predicates
+        p_annotation = types.ROOT_PREDICATE.child(ns.bsfs.Annotation)
+        p_tag = types.ROOT_PREDICATE.child(ns.bse.tag, domain=n_ent, range=n_tag)
+        p_group = p_tag.child(ns.bse.group, domain=n_image, unique=True)
+        p_comment = p_annotation.child(ns.bse.comment, range=l_string)
+        # features
+        f_colors = types.ROOT_FEATURE.child(URI('https://schema.bsfs.io/core/Feature/colors_spatial'),
+            dtype=ns.bsfs.dtype().f16, distance=ns.bsfs.euclidean)
+        f_colors1234 = f_colors.child(URI('https://schema.bsfs.io/core/Feature/colors_spatial#1234'), dimension=1024)
+        f_colors4321 = f_colors.child(URI('https://schema.bsfs.io/core/Feature/colors_spatial#4321'), dimension=2048)
+        # schema
+        ref = Schema(
+            {p_annotation, p_tag, p_group, p_comment},
+            {n_ent, n_tag, n_image},
+            {l_string, l_integer, l_boolean, f_colors, f_colors1234, f_colors4321})
+        # load from string
+        gen = from_string('''
+            # generic prefixes
+            prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
+            prefix xsd: <http://www.w3.org/2001/XMLSchema#>
+
+            # bsfs prefixes
+            prefix bsfs: <https://schema.bsfs.io/core/>
+            prefix bse: <https://schema.bsfs.io/core/Entity#>
+            prefix bsl: <https://schema.bsfs.io/core/Literal/>
+            prefix bsa: <https://schema.bsfs.io/core/Literal/Array/>
+
+            # nodes
+            bsfs:Entity rdfs:subClassOf bsfs:Node ;
+                rdfs:label "Principal node"^^xsd:string .
+            bsfs:Tag rdfs:subClassOf bsfs:Node ;
+                rdfs:label "Tag"^^xsd:string .
+            bsfs:Image rdfs:subClassOf bsfs:Entity .
+
+            # literals
+            xsd:string rdfs:subClassOf bsfs:Literal ;
+                rdfs:label "A sequence of characters"^^xsd:string .
+            bsl:Array rdfs:subClassOf bsfs:Literal .
+            bsa:Feature rdfs:subClassOf bsl:Array.
+            bsl:Number rdfs:subClassOf bsfs:Literal .
+            xsd:integer rdfs:subClassOf bsl:Number .
+            xsd:boolean rdfs:subClassOf bsfs:Literal .
+
+
+            # abstract predicates
+            bsfs:Annotation rdfs:subClassOf bsfs:Predicate ;
+                rdfs:label "node annotation"^^xsd:string .
+
+            # feature instances
+            <https://schema.bsfs.io/core/Feature/colors_spatial> rdfs:subClassOf bsa:Feature ;
+                bsfs:dtype <https://schema.bsfs.io/core/dtype#f16> ;
+                bsfs:distance bsfs:euclidean ;
+                # annotations
+                rdfs:label "ColorsSpatial instances. Dimension depends on instance."^^xsd:string ;
+                bsfs:first_arg "1234"^^xsd:integer ;
+                bsfs:second_arg "hello world"^^xsd:string .
+
+            <https://schema.bsfs.io/core/Feature/colors_spatial#1234> rdfs:subClassOf <https://schema.bsfs.io/core/Feature/colors_spatial> ;
+                bsfs:dimension "1024"^^xsd:integer ;
+                rdfs:label "Main colors spatial instance"^^xsd:string .
+
+            <https://schema.bsfs.io/core/Feature/colors_spatial#4321> rdfs:subClassOf <https://schema.bsfs.io/core/Feature/colors_spatial> ;
+                bsfs:dimension "2048"^^xsd:integer .
+
+            # predicate instances
+            bse:tag rdfs:subClassOf bsfs:Predicate ;
+                rdfs:domain bsfs:Entity ;
+                rdfs:range bsfs:Tag ;
+                bsfs:unique "false"^^xsd:boolean ;
+                # annotations
+                rdfs:label "connect entity to a tag"^^xsd:string .
+
+            bse:group rdfs:subClassOf bse:tag ; # subtype of another predicate
+                rdfs:domain bsfs:Image ;
+                bsfs:unique "true"^^xsd:boolean .
+
+            bse:comment rdfs:subClassOf bsfs:Annotation ; # subtype of abstract predicate
+                rdfs:domain bsfs:Node ;
+                rdfs:range xsd:string ;
+                bsfs:unique "false"^^xsd:boolean .
+
+            ''')
+        # schemas are equal
+        self.assertEqual(ref, gen)
+        # check annotations
+        self.assertDictEqual(gen.node(ns.bsfs.Entity).annotations, {ns.rdfs.label: 'Principal node'})
+        self.assertDictEqual(gen.node(ns.bsfs.Tag).annotations, {ns.rdfs.label: 'Tag'})
+        self.assertDictEqual(gen.literal(ns.xsd.string).annotations, {ns.rdfs.label: 'A sequence of characters'})
+        self.assertDictEqual(gen.predicate(ns.bsfs.Annotation).annotations, {ns.rdfs.label: 'node annotation'})
+        self.assertDictEqual(gen.literal(URI('https://schema.bsfs.io/core/Feature/colors_spatial')).annotations, {
+            ns.rdfs.label: 'ColorsSpatial instances. Dimension depends on instance.',
+            ns.bsfs.first_arg: 1234,
+            ns.bsfs.second_arg: 'hello world',
+            })
+        self.assertDictEqual(gen.literal(URI('https://schema.bsfs.io/core/Feature/colors_spatial#1234')).annotations, {
+            ns.rdfs.label: 'Main colors spatial instance'})
+        self.assertDictEqual(gen.predicate(ns.bse.tag).annotations, {ns.rdfs.label: 'connect entity to a tag'})
+
+        # blank nodes result in an error
+        self.assertRaises(errors.BackendError, from_string, '''
+            prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
+            prefix bsfs: <https://schema.bsfs.io/core/>
+            bsfs:Entity rdfs:subClassOf bsfs:Node ;
+                bsfs:foo _:bar .
+            ''')
+
+
+class TestToString(unittest.TestCase):
+
+    def test_empty(self):
+        self.assertEqual(Schema(), from_string(to_string(Schema())))
+
+    def test_parse(self):
+        schema = Schema()
+        schema._nodes[ns.bsfs.Invalid] = 123 # NOTE: Access protected to force an invalid schema
+        self.assertRaises(TypeError, to_string, schema)
+
+    def test_literal(self):
+        # root literals
+        l_str = types.ROOT_LITERAL.child(ns.xsd.string)
+        # derived literals
+        l_int = types.ROOT_NUMBER.child(ns.xsd.integer)
+        l_unsigned = l_int.child(ns.xsd.unsigned)
+        # create schema
+        schema = Schema(literals={l_int, l_str, l_unsigned})
+
+        schema_str = to_string(schema)
+        # all symbols are serialized
+        self.assertIn('xsd:string', schema_str)
+        self.assertIn('xsd:integer', schema_str)
+        self.assertIn('xsd:unsigned', schema_str)
+        # unserialize yields the original schema
+        self.assertEqual(schema, from_string(schema_str))
+
+        # literals that have no parent are ignored
+        schema = Schema(literals={types.Literal(ns.bsfs.Invalid, None)})
+        self.assertEqual(Schema(), from_string(to_string(schema)))
+        self.assertNotIn('Invalid', to_string(schema))
+
+        # literal annotations are serialized
+        annotations = {
+            ns.rdfs.label: 'hello world',
+            ns.schema.description: 'some text',
+            ns.bsfs.foo: 1234,
+            ns.bsfs.bar: True,
+            }
+        l_str = types.ROOT_LITERAL.child(ns.xsd.string, **annotations)
+        self.assertDictEqual(
+            annotations,
+            from_string(to_string(Schema(literals={l_str}))).literal(ns.xsd.string).annotations)
+
+
+    def test_node(self):
+        # root nodes
+        n_ent = types.ROOT_NODE.child(ns.bsfs.Entity)
+        n_tag = types.ROOT_NODE.child(ns.bsfs.Tag)
+        # derived nodes
+        n_img = n_ent.child(ns.bsfs.Image)
+        n_doc = n_ent.child(ns.bsfs.Document)
+        n_grp = n_tag.child(ns.bsfs.Group)
+        # create schema
+        schema = Schema(nodes={n_ent, n_img, n_doc, n_tag, n_grp})
+
+        schema_str = to_string(schema)
+        # all symbols are serialized
+        self.assertIn('bsfs:Entity', schema_str)
+        self.assertIn('bsfs:Tag', schema_str)
+        self.assertIn('bsfs:Image', schema_str)
+        self.assertIn('bsfs:Document', schema_str)
+        self.assertIn('bsfs:Group', schema_str)
+        # unserialize yields the original schema
+        self.assertEqual(schema, from_string(schema_str))
+
+        # nodes that have no parent are ignored
+        schema = Schema(nodes={types.Node(ns.bsfs.Invalid, None)})
+        self.assertEqual(Schema(), from_string(to_string(schema)))
+        self.assertNotIn('Invalid', to_string(schema))
+
+        # node annotations are serialized
+        annotations = {
+            ns.rdfs.label: 'hello world',
+            ns.schema.description: 'some text',
+            ns.bsfs.foo: 1234,
+            ns.bsfs.bar: True,
+            }
+        n_ent = types.ROOT_NODE.child(ns.bsfs.Entity, **annotations)
+        self.assertDictEqual(
+            annotations,
+            from_string(to_string(Schema(nodes={n_ent}))).node(ns.bsfs.Entity).annotations)
+
+
+    def test_predicate(self):
+        # auxiliary types
+        n_ent = types.ROOT_NODE.child(ns.bsfs.Entity)
+        l_str = types.ROOT_LITERAL.child(ns.xsd.string)
+        # root predicates
+        p_annotation = types.ROOT_PREDICATE.child(ns.bsfs.Annotation, domain=n_ent)
+        p_owner = types.ROOT_PREDICATE.child(ns.bse.owner, range=l_str, unique=True)
+        # derived predicates
+        p_comment = p_annotation.child(ns.bse.comment, range=l_str) # inherits domain
+        p_note = p_comment.child(ns.bse.note, unique=True) # inherits domain/range
+        # create schema
+        schema = Schema({p_owner, p_comment, p_note})
+
+        schema_str = to_string(schema)
+        # all symbols are serialized
+        self.assertIn('bsfs:Entity', schema_str)
+        self.assertIn('xsd:string', schema_str)
+        self.assertIn('bsfs:Annotation', schema_str)
+        self.assertIn('bse:comment', schema_str)
+        self.assertIn('bse:owner', schema_str)
+        self.assertIn('bse:note', schema_str)
+        # inherited properties are not serialized
+        self.assertIsNotNone(re.search(r'bse:comment[^\.]*rdfs:range[^\.]', schema_str))
+        self.assertIsNone(re.search(r'bse:comment[^\.]*rdfs:domain[^\.]', schema_str))
+        #p_note has no domain/range
+        self.assertIsNone(re.search(r'bse:note[^\.]*rdfs:domain[^\.]', schema_str))
+        self.assertIsNone(re.search(r'bse:note[^\.]*rdfs:range[^\.]', schema_str))
+        # unserialize yields the original schema
+        self.assertEqual(schema, from_string(schema_str))
+
+        # predicate annotations are serialized
+        annotations = {
+            ns.rdfs.label: 'hello world',
+            ns.schema.description: 'some text',
+            ns.bsfs.foo: 1234,
+            ns.bsfs.bar: False,
+            }
+        p_annotation = types.ROOT_PREDICATE.child(ns.bsfs.Annotation, **annotations)
+        self.assertDictEqual(
+            annotations,
+            from_string(to_string(Schema({p_annotation}))).predicate(ns.bsfs.Annotation).annotations)
+
+
+    def test_feature(self):
+        # root features
+        f_colors = types.ROOT_FEATURE.child(URI('https://schema.bsfs.io/core/Feature/colors'),
+            distance=ns.bsfs.cosine)
+        # derived features
+        f_colors1234 = f_colors.child(URI('https://schema.bsfs.io/core/Feature/colors#1234'),
+            dimension=1024) # inherits dtype, distance
+        f_colors4321 = f_colors.child(URI('https://schema.bsfs.io/core/Feature/colors#4321'),
+            dimension=2048, distance=ns.bsfs.euclidean) # inherits dtype
+        # create schema
+        schema = Schema(literals={f_colors, f_colors1234, f_colors4321})
+
+        schema_str = to_string(schema)
+        # all symbols are serialized
+        self.assertIn('bsl:Array', schema_str)
+        self.assertIn('<https://schema.bsfs.io/core/Feature/colors', schema_str)
+        self.assertIn('<https://schema.bsfs.io/core/Feature/colors#1234', schema_str)
+        self.assertIn('<https://schema.bsfs.io/core/Feature/colors#4321', schema_str)
+        # inherited properties are not serialized
+        self.assertIsNotNone(re.search(r'<https://schema.bsfs\.io/core/Feature/colors#1234>.*[^\.]*bsfs:dimension[^\.]', schema_str))
+        self.assertIsNone(re.search(r'<https://schema\.bsfs\.io/core/Feature/colors#1234>.*[^\.]*bsfs:dtype[^\.]', schema_str))
+        self.assertIsNone(re.search(r'<https://schema\.bsfs\.io/core/Feature/colors#1234>.*[^\.]*bsfs:distance[^\.]', schema_str))
+        self.assertIsNotNone(re.search(r'<https://schema\.bsfs\.io/core/Feature/colors#4321>.*[^\.]*bsfs:dimension[^\.]', schema_str))
+        self.assertIsNotNone(re.search(r'<https://schema\.bsfs\.io/core/Feature/colors#4321>.*[^\.]*bsfs:distance[^\.]', schema_str))
+        self.assertIsNone(re.search(r'<https://schema\.bsfs\.io/core/Feature/colors#4321>.*[^\.]*bsfs:dtype[^\.]', schema_str))
+        # unserialize yields the original schema
+        self.assertEqual(schema, from_string(schema_str))
+
+        # predicate annotations are serialized
+        annotations = {
+            ns.rdfs.label: 'hello world',
+            ns.schema.description: 'some text',
+            ns.bsfs.foo: 1234,
+            ns.bsfs.bar: False,
+            }
+        f_colors = types.ROOT_FEATURE.child(URI('https://schema.bsfs.io/core/Feature/colors'),
+            dtype=ns.bsfs.dtype().f16, distance=ns.bsfs.euclidean,
+            **annotations)
+        self.assertDictEqual(
+            annotations,
+            from_string(to_string(Schema(literals={f_colors}))).literal(URI('https://schema.bsfs.io/core/Feature/colors')).annotations)
+
+
+## main ##
+
+if __name__ == '__main__':
+    unittest.main()
+
+## EOF ##
diff --git a/test/schema/test_types.py b/test/schema/test_types.py
index 4a49e6e..9bfa8c5 100644
--- a/test/schema/test_types.py
+++ b/test/schema/test_types.py
@@ -1,24 +1,23 @@
-"""
 
-Part of the tagit test suite.
-A copy of the license is provided with the project.
-Author: Matthias Baumgartner, 2022
-"""
 # imports
 import operator
 import unittest
 
 # bsfs imports
 from bsfs.namespace import ns
+from bsfs.schema.types import ROOT_PREDICATE, ROOT_VERTEX, ROOT_FEATURE
 from bsfs.utils import errors
 
 # objects to test
-from bsfs.schema.types import _Type, _Vertex, Node, Literal, Predicate
+from bsfs.schema.types import _Type, Vertex, Node, Literal, Predicate, Feature
 
 
 ## code ##
 
+ns.bse = ns.bsfs.Entity()
+
 class TestType(unittest.TestCase):
+
     def test_parents(self):
         # create some types
         fst = _Type('First')
@@ -31,7 +30,25 @@ class TestType(unittest.TestCase):
         self.assertListEqual(list(trd.parents()), [snd, fst])
         self.assertListEqual(list(frd.parents()), [trd, snd, fst])
 
-    def test_essentials(self):
+    def test_annotations(self):
+        # annotations can be empty
+        self.assertDictEqual(_Type('Foo', None).annotations, {})
+        # annotations are stored
+        self.assertDictEqual(_Type('Foo', None, foo='bar', bar=123).annotations, {
+            'foo': 'bar',
+            'bar': 123})
+        # comparison ignores annotations
+        self.assertEqual(
+            _Type('Foo', None, foo='bar', bar='foo'),
+            _Type('Foo', None, hello='world', foobar=1234))
+        self.assertEqual(
+            hash(_Type('Foo', None, foo='bar', bar='foo')),
+            hash(_Type('Foo', None, hello='world', foobar=1234)))
+        # annotations can be passed to child
+        self.assertDictEqual(_Type('First', foo='bar').child('Second', bar='foo').annotations, {
+            'bar': 'foo'})
+
+    def test_string_conversion(self):
         # type w/o parent
         self.assertEqual(str(_Type('Foo')), '_Type(Foo)')
         self.assertEqual(repr(_Type('Foo')), '_Type(Foo, None)')
@@ -51,14 +68,17 @@ class TestType(unittest.TestCase):
         self.assertEqual(str(_Type('Foo', SubType('Bar'))), '_Type(Foo)')
         self.assertEqual(repr(_Type('Foo', SubType('Bar'))), '_Type(Foo, SubType(Bar, None))')
 
-    def test_get_child(self):
+    def test_child(self):
         # callee is used as parent
-        self.assertEqual(_Type('First').get_child('Second'), _Type('Second', _Type('First')))
+        self.assertEqual(_Type('First').child('Second'), _Type('Second', _Type('First')))
         # works with multiple parents
-        self.assertEqual(_Type('First').get_child('Second').get_child('Third'), _Type('Third', _Type('Second', _Type('First'))))
+        self.assertEqual(_Type('First').child('Second').child('Third'), _Type('Third', _Type('Second', _Type('First'))))
         # type persists
         class Foo(_Type): pass
-        self.assertEqual(Foo('First').get_child('Second'), Foo('Second', Foo('First')))
+        self.assertEqual(Foo('First').child('Second'), Foo('Second', Foo('First')))
+        # annotations are respected
+        self.assertDictEqual(_Type('First', foo='bar').child('Second', bar='foo').annotations, {
+            'bar': 'foo'})
 
     def test_equality(self):
         # equality depends on uri
@@ -76,6 +96,13 @@ class TestType(unittest.TestCase):
         # comparison respects parent
         self.assertNotEqual(_Type('Foo', _Type('Bar')), _Type('Foo'))
         self.assertNotEqual(hash(_Type('Foo', _Type('Bar'))), hash(_Type('Foo')))
+        # comparison ignores annotations
+        self.assertEqual(
+            _Type('Foo', None, foo='bar', bar='foo'),
+            _Type('Foo', None, hello='world', foobar=1234))
+        self.assertEqual(
+            hash(_Type('Foo', None, foo='bar', bar='foo')),
+            hash(_Type('Foo', None, hello='world', foobar=1234)))
 
     def test_order(self):
         # create some types.
@@ -109,27 +136,43 @@ class TestType(unittest.TestCase):
         self.assertFalse(bike > bicycle)
         self.assertFalse(bike >= bicycle)
         self.assertFalse(bike == bicycle)
+
+        # comparing different classes returns False ...
+        # ... when classes are hierarchically related
         class Foo(_Type): pass
-        foo = Foo(bike.uri, bike.parent)
-        # cannot compare different types
-        self.assertRaises(TypeError, operator.lt, foo, bike)
-        self.assertRaises(TypeError, operator.le, foo, bike)
-        self.assertRaises(TypeError, operator.gt, foo, bike)
-        self.assertRaises(TypeError, operator.ge, foo, bike)
+        foo = Foo('Foo', bike)
+        self.assertFalse(foo < bike)
+        self.assertFalse(foo <= bike)
+        self.assertFalse(foo > bike)
+        self.assertFalse(foo >= bike)
         # goes both ways
-        self.assertRaises(TypeError, operator.lt, bike, foo)
-        self.assertRaises(TypeError, operator.le, bike, foo)
-        self.assertRaises(TypeError, operator.gt, bike, foo)
-        self.assertRaises(TypeError, operator.ge, bike, foo)
+        self.assertFalse(bike < foo)
+        self.assertFalse(bike <= foo)
+        self.assertFalse(bike > foo)
+        self.assertFalse(bike >= foo)
+        # ... when classes are unrelated
+        class Bar(_Type): pass
+        bar = Bar('Bar', bike)
+        self.assertFalse(foo < bar)
+        self.assertFalse(foo <= bar)
+        self.assertFalse(foo > bar)
+        self.assertFalse(foo >= bar)
+        # goes both ways
+        self.assertFalse(bar < foo)
+        self.assertFalse(bar <= foo)
+        self.assertFalse(bar > foo)
+        self.assertFalse(bar >= foo)
+
 
 class TestPredicate(unittest.TestCase):
     def test_construction(self):
         # domain must be a node
         self.assertRaises(TypeError, Predicate, ns.bse.foo, 1234, None, True)
         self.assertRaises(TypeError, Predicate, ns.bse.foo, None, Literal(ns.bsfs.Foo, None), None, True)
-        # range must be None, a Literal, or a Node
+        # range must be a Literal, a Node, or the root Vertex
+        self.assertRaises(TypeError, Predicate, ns.bse.foo, None, Node(ns.bsfs.Node, None), None, True)
         self.assertRaises(TypeError, Predicate, ns.bse.foo, None, Node(ns.bsfs.Node, None), 1234, True)
-        self.assertRaises(TypeError, Predicate, ns.bse.foo, None, Node(ns.bsfs.Node, None), _Vertex(ns.bsfs.Foo, None), True)
+        self.assertRaises(TypeError, Predicate, ns.bse.foo, None, Node(ns.bsfs.Node, None), Vertex(ns.bsfs.Foo, None), True)
         self.assertRaises(TypeError, Predicate, ns.bse.foo, None, Node(ns.bsfs.Node, None), _Type(ns.bsfs.Foo, None), True)
         class Foo(): pass
         self.assertRaises(TypeError, Predicate, ns.bse.foo, None, Node(ns.bsfs.Node, None), Foo(), True)
@@ -138,82 +181,160 @@ class TestPredicate(unittest.TestCase):
         n_root = Node(ns.bsfs.Node, None)
         n_ent = Node(ns.bsfs.Entity, Node(ns.bsfs.Node, None))
         n_tag = Node(ns.bsfs.Tag, Node(ns.bsfs.Tag, None))
-        root = Predicate(
-            uri=ns.bsfs.Predicate,
-            parent=None,
+        root = ROOT_PREDICATE
+        tag = Predicate(
+            uri=ns.bse.tag,
+            parent=root,
             domain=n_root,
-            range=None,
+            range=n_tag,
             unique=False,
             )
         # instance is equal to itself
-        self.assertEqual(root, root)
-        self.assertEqual(hash(root), hash(root))
+        self.assertEqual(tag, tag)
+        self.assertEqual(hash(tag), hash(tag))
         # instance is equal to a clone
-        self.assertEqual(root, Predicate(ns.bsfs.Predicate, None, n_root, None, False))
-        self.assertEqual(hash(root), hash(Predicate(ns.bsfs.Predicate, None, n_root, None, False)))
+        self.assertEqual(tag, Predicate(ns.bse.tag, root, n_root, n_tag, False))
+        self.assertEqual(hash(tag), hash(Predicate(ns.bse.tag, root, n_root, n_tag, False)))
         # equality respects uri
-        self.assertNotEqual(root, Predicate(ns.bsfs.Alternative, None, n_root, None, False))
-        self.assertNotEqual(hash(root), hash(Predicate(ns.bsfs.Alternative, None, n_root, None, False)))
+        self.assertNotEqual(tag, Predicate(ns.bsfs.Alternative, root, n_root, n_tag, False))
+        self.assertNotEqual(hash(tag), hash(Predicate(ns.bsfs.Alternative, root, n_root, n_tag, False)))
         # equality respects parent
-        self.assertNotEqual(root, Predicate(ns.bsfs.Predicate, n_root, n_root, None, False))
-        self.assertNotEqual(hash(root), hash(Predicate(ns.bsfs.Predicate, n_root, n_root, None, False)))
+        self.assertNotEqual(tag, Predicate(ns.bse.tag, n_root, n_root, n_tag, False))
+        self.assertNotEqual(hash(tag), hash(Predicate(ns.bse.tag, n_root, n_root, n_tag, False)))
         # equality respects domain
-        self.assertNotEqual(root, Predicate(ns.bsfs.Predicate, None, n_ent, None, False))
-        self.assertNotEqual(hash(root), hash(Predicate(ns.bsfs.Predicate, None, n_ent, None, False)))
+        self.assertNotEqual(tag, Predicate(ns.bse.tag, root, n_ent, n_tag, False))
+        self.assertNotEqual(hash(tag), hash(Predicate(ns.bse.tag, root, n_ent, n_tag, False)))
         # equality respects range
-        self.assertNotEqual(root, Predicate(ns.bsfs.Predicate, None, n_root, n_root, False))
-        self.assertNotEqual(hash(root), hash(Predicate(ns.bsfs.Predicate, None, n_root, n_root, False)))
+        self.assertNotEqual(tag, Predicate(ns.bse.tag, root, n_root, n_root, False))
+        self.assertNotEqual(hash(tag), hash(Predicate(ns.bse.tag, root, n_root, n_root, False)))
         # equality respects unique
-        self.assertNotEqual(root, Predicate(ns.bsfs.Predicate, None, n_root, None, True))
-        self.assertNotEqual(hash(root), hash(Predicate(ns.bsfs.Predicate, None, n_root, None, True)))
+        self.assertNotEqual(tag, Predicate(ns.bse.tag, root, n_root, n_tag, True))
+        self.assertNotEqual(hash(tag), hash(Predicate(ns.bse.tag, root, n_root, n_tag, True)))
 
-    def test_get_child(self):
+    def test_child(self):
         n_root = Node(ns.bsfs.Node, None)
+        l_root = Literal(ns.bsfs.Literal, None)
         n_ent = Node(ns.bsfs.Entity, Node(ns.bsfs.Node, None))
         n_tag = Node(ns.bsfs.Tag, Node(ns.bsfs.Tag, None))
-        root = Predicate(
-            uri=ns.bsfs.Predicate,
-            parent=None,
-            domain=n_root,
-            range=None,
-            unique=False,
-            )
+        root = ROOT_PREDICATE
         tag = Predicate(
-            uri=ns.bsfs.Entity,
+            uri=ns.bse.tag,
             parent=root,
             domain=n_ent,
             range=n_tag,
             unique=False,
             )
 
+        # child returns Predicate
+        self.assertIsInstance(tag.child(ns.bse.foo), Predicate)
         # uri is respected
-        self.assertEqual(ns.bse.foo, tag.get_child(ns.bse.foo).uri)
+        self.assertEqual(ns.bse.foo, tag.child(ns.bse.foo).uri)
         # domain is respected
         dom = Node(ns.bsfs.Image, n_ent)
-        self.assertEqual(dom, tag.get_child(ns.bse.foo, domain=dom).domain)
+        self.assertEqual(dom, tag.child(ns.bse.foo, domain=dom).domain)
         # range is respected
         rng = Node(ns.bsfs.Group, n_tag)
-        self.assertEqual(rng, tag.get_child(ns.bse.foo, range=rng).range)
+        self.assertEqual(rng, tag.child(ns.bse.foo, range=rng).range)
         # cannot set range to None
-        self.assertEqual(n_tag, tag.get_child(ns.bse.foo, range=None).range)
+        self.assertEqual(n_tag, tag.child(ns.bse.foo, range=None).range)
         # unique is respected
-        self.assertTrue(tag.get_child(ns.bse.foo, unique=True).unique)
+        self.assertTrue(tag.child(ns.bse.foo, unique=True).unique)
+        # annotations are respected
+        self.assertDictEqual(tag.child(ns.bse.foo, foo='bar', bar=123).annotations, {
+            'foo': 'bar',
+            'bar': 123,
+            })
 
         # domain is inherited from parent
-        self.assertEqual(n_ent, tag.get_child(ns.bse.foo).domain)
+        self.assertEqual(n_root, root.child(ns.bse.foo).domain)
+        self.assertEqual(n_ent, tag.child(ns.bse.foo).domain)
         # range is inherited from parent
-        self.assertEqual(n_tag, tag.get_child(ns.bse.foo).range)
+        self.assertEqual(ROOT_VERTEX, root.child(ns.bse.foo).range)
+        self.assertEqual(n_tag, tag.child(ns.bse.foo).range)
         # uniqueness is inherited from parent
-        self.assertFalse(tag.get_child(ns.bse.foo).unique)
+        self.assertFalse(tag.child(ns.bse.foo).unique)
 
         # domain must be subtype of parent's domain
-        self.assertRaises(errors.ConsistencyError, tag.get_child, ns.bse.foo, domain=n_root)
-        self.assertRaises(errors.ConsistencyError, tag.get_child, ns.bse.foo, domain=Node(ns.bsfs.Image, n_root))
-        # range cannot be None
-        self.assertRaises(ValueError, root.get_child, ns.bse.foo)
+        self.assertRaises(errors.ConsistencyError, tag.child, ns.bse.foo, domain=n_root)
+        self.assertRaises(errors.ConsistencyError, tag.child, ns.bse.foo, domain=Node(ns.bsfs.Image, n_root))
         # range must be subtype of parent's range
-        self.assertRaises(errors.ConsistencyError, tag.get_child, ns.bse.foo, range=n_root)
-        self.assertRaises(errors.ConsistencyError, tag.get_child, ns.bse.foo, range=Node(ns.bsfs.Image, n_root))
+        self.assertRaises(errors.ConsistencyError, tag.child, ns.bse.foo, range=n_root)
+        self.assertRaises(errors.ConsistencyError, tag.child, ns.bse.foo, range=Node(ns.bsfs.Image, n_root))
+        self.assertRaises(errors.ConsistencyError, tag.child, ns.bse.foo, range=Literal(ns.bsfs.Tag, l_root))
+        # range can be subtyped from ROOT_VERTEX to Node or Literal
+        self.assertEqual(n_root, root.child(ns.bse.foo, range=n_root).range)
+        self.assertEqual(l_root, root.child(ns.bse.foo, range=l_root).range)
+
+
+class TestFeature(unittest.TestCase):
+    def test_construction(self):
+        n_root = Node(ns.bsfs.Node, None)
+        l_root = Literal(ns.bsfs.Literal, None)
+        # dimension, dtype, and distance are respected
+        feat = Feature(ns.bsfs.Feature, None, 1234, ns.bsfs.float, ns.bsfs.euclidean)
+        self.assertEqual(1234, feat.dimension)
+        self.assertEqual(ns.bsfs.float, feat.dtype)
+        self.assertEqual(ns.bsfs.euclidean, feat.distance)
+
+    def test_equality(self):
+        n_ent = Node(ns.bsfs.Entity, Node(ns.bsfs.Node, None))
+        colors = Feature(
+            uri=ns.bse.colors,
+            parent=ROOT_FEATURE,
+            dimension=1234,
+            dtype=ns.bsfs.float,
+            distance=ns.bsfs.euclidean,
+            )
+        # instance is equal to itself
+        self.assertEqual(colors, colors)
+        self.assertEqual(hash(colors), hash(colors))
+        # instance is equal to a clone
+        self.assertEqual(colors, Feature(ns.bse.colors, ROOT_FEATURE, 1234, ns.bsfs.float, ns.bsfs.euclidean))
+        self.assertEqual(hash(colors), hash(Feature(ns.bse.colors, ROOT_FEATURE, 1234, ns.bsfs.float, ns.bsfs.euclidean)))
+        # equality respects dimension
+        self.assertNotEqual(colors, Feature(ns.bse.colors, ROOT_FEATURE, 4321, ns.bsfs.float, ns.bsfs.euclidean))
+        self.assertNotEqual(hash(colors), hash(Feature(ns.bse.colors, ROOT_FEATURE, 4321, ns.bsfs.float, ns.bsfs.euclidean)))
+        # equality respects dtype
+        self.assertNotEqual(colors, Feature(ns.bse.colors, ROOT_FEATURE, 1234, ns.bsfs.integer, ns.bsfs.euclidean))
+        self.assertNotEqual(hash(colors), hash(Feature(ns.bse.colors, ROOT_FEATURE, 1234, ns.bsfs.integer, ns.bsfs.euclidean)))
+        # equality respects distance
+        self.assertNotEqual(colors, Feature(ns.bse.colors, ROOT_FEATURE, 1234, ns.bsfs.float, ns.bsfs.cosine))
+        self.assertNotEqual(hash(colors), hash(Feature(ns.bse.colors, ROOT_FEATURE, 1234, ns.bsfs.float, ns.bsfs.cosine)))
+
+    def test_child(self):
+        n_root = Node(ns.bsfs.Node, None)
+        n_ent = Node(ns.bsfs.Entity, n_root)
+        l_root = Literal(ns.bsfs.Literal, None)
+        colors = Feature(
+            uri=ns.bse.colors,
+            parent=ROOT_FEATURE,
+            dimension=1234,
+            dtype=ns.bsfs.float,
+            distance=ns.bsfs.euclidean,
+            )
+
+        # child returns Feature
+        self.assertIsInstance(colors.child(ns.bse.foo), Feature)
+        # uri is respected
+        self.assertEqual(ns.bse.foo, colors.child(ns.bse.foo).uri)
+        # dimension is respected
+        self.assertEqual(4321, colors.child(ns.bse.foo, dimension=4321).dimension)
+        # dtype is respected
+        self.assertEqual(ns.bsfs.integer, colors.child(ns.bse.foo, dtype=ns.bsfs.integer).dtype)
+        # distance is respected
+        self.assertEqual(ns.bsfs.cosine, colors.child(ns.bse.foo, distance=ns.bsfs.cosine).distance)
+        # annotations are respected
+        self.assertDictEqual(colors.child(ns.bse.foo, foo='bar', bar=123).annotations, {
+            'foo': 'bar',
+            'bar': 123,
+            })
+
+        # dimension is inherited from parent
+        self.assertEqual(1234, colors.child(ns.bse.foo).dimension)
+        # dtype is inherited from parent
+        self.assertEqual(ns.bsfs.float, colors.child(ns.bse.foo).dtype)
+        # distance is inherited from parent
+        self.assertEqual(ns.bsfs.euclidean, colors.child(ns.bse.foo).distance)
 
 
 ## main ##
@@ -222,4 +343,3 @@ if __name__ == '__main__':
     unittest.main()
 
 ## EOF ##
-
diff --git a/test/triple_store/sparql/__init__.py b/test/triple_store/sparql/__init__.py
new file mode 100644
index 0000000..e69de29
--- /dev/null
+++ b/test/triple_store/sparql/__init__.py
diff --git a/test/triple_store/sparql/test_distance.py b/test/triple_store/sparql/test_distance.py
new file mode 100644
index 0000000..e95be5a
--- /dev/null
+++ b/test/triple_store/sparql/test_distance.py
@@ -0,0 +1,56 @@
+
+# imports
+import numpy as np
+import unittest
+
+# objects to test
+from bsfs.triple_store.sparql import distance
+
+
+## code ##
+
+class TestDistance(unittest.TestCase):
+
+    def test_euclid(self):
+        # self-distance is zero
+        self.assertEqual(distance.euclid([1,2,3,4], [1,2,3,4]), 0.0)
+        # accepts list-like arguments
+        self.assertAlmostEqual(distance.euclid([1,2,3,4], [2,3,4,5]), 2.0, 3)
+        self.assertAlmostEqual(distance.euclid((1,2,3,4), (2,3,4,5)), 2.0, 3)
+        # dimension can vary
+        self.assertAlmostEqual(distance.euclid([1,2,3], [2,3,4]), 1.732, 3)
+        self.assertAlmostEqual(distance.euclid([1,2,3,4,5], [2,3,4,5,6]), 2.236, 3)
+        # vector can be zero
+        self.assertAlmostEqual(distance.euclid([0,0,0], [1,2,3]), 3.742, 3)
+
+    def test_cosine(self):
+        # self-distance is zero
+        self.assertEqual(distance.cosine([1,2,3,4], [1,2,3,4]), 0.0)
+        # accepts list-like arguments
+        self.assertAlmostEqual(distance.cosine([1,2,3,4], [4,3,2,1]), 0.333, 3)
+        self.assertAlmostEqual(distance.cosine((1,2,3,4), (4,3,2,1)), 0.333, 3)
+        # dimension can vary
+        self.assertAlmostEqual(distance.cosine([1,2,3], [3,2,1]), 0.286, 3)
+        self.assertAlmostEqual(distance.cosine([1,2,3,4,5], [5,4,3,2,1]), 0.364, 3)
+        # vector can be zero
+        self.assertAlmostEqual(distance.cosine([0,0,0], [1,2,3]), 1.0, 3)
+
+    def test_manhatten(self):
+        # self-distance is zero
+        self.assertEqual(distance.manhatten([1,2,3,4], [1,2,3,4]), 0.0)
+        # accepts list-like arguments
+        self.assertAlmostEqual(distance.manhatten([1,2,3,4], [2,3,4,5]), 4.0, 3)
+        self.assertAlmostEqual(distance.manhatten((1,2,3,4), (2,3,4,5)), 4.0, 3)
+        # dimension can vary
+        self.assertAlmostEqual(distance.manhatten([1,2,3], [2,3,4]), 3.0, 3)
+        self.assertAlmostEqual(distance.manhatten([1,2,3,4,5], [2,3,4,5,6]), 5.0, 3)
+        # vector can be zero
+        self.assertAlmostEqual(distance.manhatten([0,0,0], [1,2,3]), 6.0, 3)
+
+
+## main ##
+
+if __name__ == '__main__':
+    unittest.main()
+
+## EOF ##
diff --git a/test/triple_store/sparql/test_parse_fetch.py b/test/triple_store/sparql/test_parse_fetch.py
new file mode 100644
index 0000000..1d793e7
--- /dev/null
+++ b/test/triple_store/sparql/test_parse_fetch.py
@@ -0,0 +1,257 @@
+
+# imports
+import rdflib
+import unittest
+
+# bsie imports
+from bsfs import schema
+from bsfs.namespace import Namespace, ns
+from bsfs.query import ast
+from bsfs.utils import errors, URI
+
+# objects to test
+from bsfs.triple_store.sparql.parse_fetch import Fetch
+
+
+## code ##
+
+ns.bse = ns.bsfs.Entity()
+ns.bst = ns.bsfs.Tag()
+ns.bsc = ns.bsfs.Collection()
+
+class TestParseFetch(unittest.TestCase):
+
+    def setUp(self):
+        self.schema = schema.from_string('''
+            prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
+            prefix xsd: <http://www.w3.org/2001/XMLSchema#>
+
+            prefix bsfs: <https://schema.bsfs.io/core/>
+            prefix bse: <https://schema.bsfs.io/core/Entity#>
+            prefix bst: <https://schema.bsfs.io/core/Tag#>
+            prefix bsc: <https://schema.bsfs.io/core/Collection#>
+
+            # nodes
+            bsfs:Entity rdfs:subClassOf bsfs:Node .
+            bsfs:Tag rdfs:subClassOf bsfs:Node .
+            bsfs:Collection rdfs:subClassOf bsfs:Node .
+
+            # literals
+            xsd:integer rdfs:subClassOf bsfs:Literal .
+            xsd:string rdfs:subClassOf bsfs:Literal .
+
+            # predicates
+            bse:tag rdfs:subClassOf bsfs:Predicate ;
+                rdfs:domain bsfs:Entity ;
+                rdfs:range bsfs:Tag .
+
+            bse:collection rdfs:subClassOf bsfs:Predicate ;
+                rdfs:domain bsfs:Entity ;
+                rdfs:range bsfs:Collection .
+
+            bse:filename rdfs:subClassOf bsfs:Predicate ;
+                rdfs:domain bsfs:Entity ;
+                rdfs:range xsd:string .
+
+            bse:rank rdfs:subClassOf bsfs:Predicate ;
+                rdfs:domain bsfs:Entity ;
+                rdfs:range xsd:integer .
+
+            bst:main rdfs:subClassOf bsfs:Predicate ;
+                rdfs:domain bsfs:Tag ;
+                rdfs:range bsfs:Entity .
+
+            bst:label rdfs:subClassOf bsfs:Predicate ;
+                rdfs:domain bsfs:Tag ;
+                rdfs:range xsd:string .
+
+            bsc:tag rdfs:subClassOf bsfs:Predicate ;
+                rdfs:domain bsfs:Collection ;
+                rdfs:range bsfs:Tag .
+
+            bsc:label rdfs:subClassOf bsfs:Predicate ;
+                rdfs:domain bsfs:Collection ;
+                rdfs:range xsd:string .
+
+            bsc:rating rdfs:subClassOf bsfs:Predicate ;
+                rdfs:domain bsfs:Collection ;
+                rdfs:range xsd:integer .
+
+            ''')
+
+        # graph to test queries
+        self.graph = rdflib.Graph()
+        # schema hierarchies
+        self.graph.add((rdflib.URIRef('https://schema.bsfs.io/core/Entity'), rdflib.RDFS.subClassOf, rdflib.URIRef('https://schema.bsfs.io/core/Node')))
+        self.graph.add((rdflib.URIRef('https://schema.bsfs.io/core/Collection'), rdflib.RDFS.subClassOf, rdflib.URIRef('https://schema.bsfs.io/core/Node')))
+        self.graph.add((rdflib.URIRef('https://schema.bsfs.io/core/Tag'), rdflib.RDFS.subClassOf, rdflib.URIRef('https://schema.bsfs.io/core/Node')))
+        # entities
+        self.graph.add((rdflib.URIRef('http://example.com/entity#1234'), rdflib.RDF.type, rdflib.URIRef('https://schema.bsfs.io/core/Entity')))
+        self.graph.add((rdflib.URIRef('http://example.com/entity#4321'), rdflib.RDF.type, rdflib.URIRef('https://schema.bsfs.io/core/Entity')))
+        # tags
+        self.graph.add((rdflib.URIRef('http://example.com/tag#1234'), rdflib.RDF.type, rdflib.URIRef('https://schema.bsfs.io/core/Tag')))
+        self.graph.add((rdflib.URIRef('http://example.com/tag#4321'), rdflib.RDF.type, rdflib.URIRef('https://schema.bsfs.io/core/Tag')))
+        # collections
+        self.graph.add((rdflib.URIRef('http://example.com/collection#1234'), rdflib.RDF.type, rdflib.URIRef('https://schema.bsfs.io/core/Collection')))
+        self.graph.add((rdflib.URIRef('http://example.com/collection#4321'), rdflib.RDF.type, rdflib.URIRef('https://schema.bsfs.io/core/Collection')))
+        # entity literals
+        self.graph.add((rdflib.URIRef('http://example.com/entity#1234'), rdflib.URIRef(ns.bse.rank), rdflib.Literal('1234', datatype=rdflib.XSD.integer)))
+        self.graph.add((rdflib.URIRef('http://example.com/entity#1234'), rdflib.URIRef(ns.bse.filename), rdflib.Literal('filename_1234', datatype=rdflib.XSD.string)))
+        #self.graph.add((rdflib.URIRef('http://example.com/entity#4321'), rdflib.URIRef(ns.bse.rank), rdflib.Literal('4321', datatype=rdflib.XSD.integer)))
+        self.graph.add((rdflib.URIRef('http://example.com/entity#4321'), rdflib.URIRef(ns.bse.filename), rdflib.Literal('filename_4321', datatype=rdflib.XSD.string)))
+        # tag literals
+        self.graph.add((rdflib.URIRef('http://example.com/tag#1234'), rdflib.URIRef(ns.bst.label), rdflib.Literal('tag_label_1234', datatype=rdflib.XSD.string)))
+        self.graph.add((rdflib.URIRef('http://example.com/tag#4321'), rdflib.URIRef(ns.bst.label), rdflib.Literal('tag_label_4321', datatype=rdflib.XSD.string)))
+        # collection literals
+        self.graph.add((rdflib.URIRef('http://example.com/collection#1234'), rdflib.URIRef(ns.bsc.label), rdflib.Literal('collection_label_1234', datatype=rdflib.XSD.string)))
+        self.graph.add((rdflib.URIRef('http://example.com/collection#1234'), rdflib.URIRef(ns.bsc.rating), rdflib.Literal('1234', datatype=rdflib.XSD.integer)))
+        self.graph.add((rdflib.URIRef('http://example.com/collection#4321'), rdflib.URIRef(ns.bsc.label), rdflib.Literal('collection_label_4321', datatype=rdflib.XSD.string)))
+        self.graph.add((rdflib.URIRef('http://example.com/collection#4321'), rdflib.URIRef(ns.bsc.rating), rdflib.Literal('4321', datatype=rdflib.XSD.integer)))
+        # entity-tag links
+        self.graph.add((rdflib.URIRef('http://example.com/entity#1234'), rdflib.URIRef(ns.bse.tag), rdflib.URIRef('http://example.com/tag#1234')))
+        self.graph.add((rdflib.URIRef('http://example.com/entity#4321'), rdflib.URIRef(ns.bse.tag), rdflib.URIRef('http://example.com/tag#4321')))
+        # entity-collection links
+        self.graph.add((rdflib.URIRef('http://example.com/entity#1234'), rdflib.URIRef(ns.bse.collection), rdflib.URIRef('http://example.com/collection#1234')))
+        self.graph.add((rdflib.URIRef('http://example.com/entity#4321'), rdflib.URIRef(ns.bse.collection), rdflib.URIRef('http://example.com/collection#4321')))
+        # collection-tag links
+        self.graph.add((rdflib.URIRef('http://example.com/collection#1234'), rdflib.URIRef(ns.bsc.tag), rdflib.URIRef('http://example.com/tag#1234')))
+        self.graph.add((rdflib.URIRef('http://example.com/collection#4321'), rdflib.URIRef(ns.bsc.tag), rdflib.URIRef('http://example.com/tag#4321')))
+        # tag-entity links # NOTE: cross-over
+        self.graph.add((rdflib.URIRef('http://example.com/tag#1234'), rdflib.URIRef(ns.bst.main), rdflib.URIRef('http://example.com/entity#4321')))
+        self.graph.add((rdflib.URIRef('http://example.com/tag#4321'), rdflib.URIRef(ns.bst.main), rdflib.URIRef('http://example.com/entity#1234')))
+
+        # default parser
+        self.parser = Fetch(self.schema)
+        self.ent = self.schema.node(ns.bsfs.Entity)
+
+
+    def test_call(self):
+        # NOTE: The individual ast components are considered in the respective tests. Here, we test __call__ specifics.
+
+        # __call__ requires a valid root type
+        self.assertRaises(errors.BackendError, self.parser, self.schema.literal(ns.bsfs.Literal), ast.fetch.This('this'))
+        self.assertRaises(errors.ConsistencyError, self.parser, self.schema.node(ns.bsfs.Node).child(ns.bsfs.Invalid), ast.fetch.This('this'))
+        # __call__ requires a parseable root
+        self.assertRaises(errors.BackendError, self.parser, self.ent, ast.filter.FilterExpression())
+        # __call__ returns an executable query
+        q = self.parser(self.ent, ast.fetch.Fetch(ns.bse.tag, ast.fetch.Value(ns.bst.label, 'label')))
+        self.assertSetEqual(set(q(self.graph)), {
+            (rdflib.URIRef('http://example.com/entity#1234'), rdflib.Literal('tag_label_1234', datatype=rdflib.XSD.string)),
+            (rdflib.URIRef('http://example.com/entity#4321'), rdflib.Literal('tag_label_4321', datatype=rdflib.XSD.string)),
+            })
+
+
+    def test_routing(self):
+        self.assertRaises(errors.BackendError, self.parser._parse_fetch_expression, self.ent, ast.fetch.FetchExpression(), '?head')
+
+
+    def test_all(self):
+        # multiple values query
+        q = self.parser(self.ent, ast.fetch.All(
+            ast.fetch.Value(ns.bse.filename, name='filename'),
+            ast.fetch.Value(ns.bse.rank, name='rank')),
+            )
+        self.assertSetEqual(set(q.names), {'filename', 'rank'})
+        if q.names == ('filename', 'rank'):
+            self.assertSetEqual(set(q(self.graph)), {
+                (rdflib.URIRef('http://example.com/entity#1234'), rdflib.Literal('filename_1234', datatype=rdflib.XSD.string), rdflib.Literal('1234', datatype=rdflib.XSD.integer)),
+                (rdflib.URIRef('http://example.com/entity#4321'), rdflib.Literal('filename_4321', datatype=rdflib.XSD.string), None),
+                })
+        else:
+            self.assertSetEqual(set(q(self.graph)), {
+                (rdflib.URIRef('http://example.com/entity#1234'), rdflib.Literal('1234', datatype=rdflib.XSD.integer), rdflib.Literal('filename_1234', datatype=rdflib.XSD.string)),
+                (rdflib.URIRef('http://example.com/entity#4321'), None,                                                rdflib.Literal('filename_4321', datatype=rdflib.XSD.string)),
+                })
+        # mixed values and node query
+        q = self.parser(self.ent, ast.fetch.All(
+            ast.fetch.Value(ns.bse.filename, name='filename'),
+            ast.fetch.Node(ns.bse.tag, name='tag'),
+            ))
+        self.assertSetEqual(set(q.names), {'filename', 'tag'})
+        if q.names == ('filename', 'tag'):
+            self.assertSetEqual(set(q(self.graph)), {
+                (rdflib.URIRef('http://example.com/entity#1234'), rdflib.Literal('filename_1234', datatype=rdflib.XSD.string), rdflib.URIRef('http://example.com/tag#1234')),
+                (rdflib.URIRef('http://example.com/entity#4321'), rdflib.Literal('filename_4321', datatype=rdflib.XSD.string), rdflib.URIRef('http://example.com/tag#4321')),
+                })
+        else:
+            self.assertSetEqual(set(q(self.graph)), {
+                (rdflib.URIRef('http://example.com/entity#1234'), rdflib.URIRef('http://example.com/tag#1234'), rdflib.Literal('filename_1234', datatype=rdflib.XSD.string)),
+                (rdflib.URIRef('http://example.com/entity#4321'), rdflib.URIRef('http://example.com/tag#4321'), rdflib.Literal('filename_4321', datatype=rdflib.XSD.string)),
+                })
+        # multiple values and second hop
+        q = self.parser(self.ent, ast.fetch.Fetch(ns.bse.tag, ast.fetch.All(
+            ast.fetch.This(name='tag'),
+            ast.fetch.Value(ns.bst.label, name='label'),
+            )))
+        self.assertSetEqual(set(q.names), {'tag', 'label'})
+        if q.names == ('tag', 'label'):
+            self.assertSetEqual(set(q(self.graph)), {
+                (rdflib.URIRef('http://example.com/entity#1234'), rdflib.URIRef('http://example.com/tag#1234'), rdflib.Literal('tag_label_1234', datatype=rdflib.XSD.string)),
+                (rdflib.URIRef('http://example.com/entity#4321'), rdflib.URIRef('http://example.com/tag#4321'), rdflib.Literal('tag_label_4321', datatype=rdflib.XSD.string)),
+                })
+        else:
+            self.assertSetEqual(set(q(self.graph)), {
+                (rdflib.URIRef('http://example.com/entity#1234'), rdflib.Literal('tag_label_1234', datatype=rdflib.XSD.string), rdflib.URIRef('http://example.com/tag#1234')),
+                (rdflib.URIRef('http://example.com/entity#4321'), rdflib.Literal('tag_label_4321', datatype=rdflib.XSD.string), rdflib.URIRef('http://example.com/tag#4321')),
+                })
+
+
+
+    def test_fetch(self):
+        # two-hop query
+        q = self.parser(self.ent, ast.fetch.Fetch(ns.bse.tag, ast.fetch.Value(ns.bst.label, 'tag_label')))
+        self.assertSetEqual(set(q(self.graph)), {
+            (rdflib.URIRef('http://example.com/entity#1234'), rdflib.Literal('tag_label_1234', datatype=rdflib.XSD.string)),
+            (rdflib.URIRef('http://example.com/entity#4321'), rdflib.Literal('tag_label_4321', datatype=rdflib.XSD.string)),
+            })
+        # three-hop-query
+        q = self.parser(self.ent, ast.fetch.Fetch(ns.bse.tag, ast.fetch.Fetch(ns.bst.main, ast.fetch.Value(ns.bse.rank, 'entity_rank'))))
+        self.assertSetEqual(set(q(self.graph)), {
+            (rdflib.URIRef('http://example.com/entity#1234'), None),
+            (rdflib.URIRef('http://example.com/entity#4321'), rdflib.Literal('1234', datatype=rdflib.XSD.integer)),
+            })
+
+
+    def test_node(self):
+        # cannot use the internal hop name
+        self.assertRaises(errors.BackendError, self.parser, self.ent, ast.fetch.Node(ns.bse.tag, self.parser.ngen.prefix[1:] + '123'))
+        # a simple Node statement
+        q = self.parser(self.ent, ast.fetch.Node(ns.bse.tag, 'tag'))
+        self.assertSetEqual(set(q.names), {'tag'})
+        self.assertSetEqual(set(q(self.graph)), {
+            (rdflib.URIRef('http://example.com/entity#1234'), rdflib.URIRef('http://example.com/tag#1234')),
+            (rdflib.URIRef('http://example.com/entity#4321'), rdflib.URIRef('http://example.com/tag#4321')),
+            })
+
+
+    def test_value(self):
+        # cannot use the internal hop name
+        self.assertRaises(errors.BackendError, self.parser, self.schema.node(ns.bsfs.Entity), ast.fetch.Value(ns.bse.filename, self.parser.ngen.prefix[1:] + '123'))
+        # a simple Value statement
+        q = self.parser(self.ent, ast.fetch.Value(ns.bse.filename, 'filename'))
+        self.assertSetEqual(set(q.names), {'filename'})
+        self.assertSetEqual(set(q(self.graph)), {
+            (rdflib.URIRef('http://example.com/entity#1234'), rdflib.Literal('filename_1234', datatype=rdflib.XSD.string)),
+            (rdflib.URIRef('http://example.com/entity#4321'), rdflib.Literal('filename_4321', datatype=rdflib.XSD.string)),
+            })
+
+
+    def test_this(self):
+        # cannot use the internal hop name
+        self.assertRaises(errors.BackendError, self.parser, self.ent, ast.fetch.This(self.parser.ngen.prefix[1:] + '123'))
+        # a simple This statement
+        self.assertEqual(self.parser._this(self.ent, ast.fetch.This('this'), '?head'),
+            ({('?head', 'this')}, ''))
+        q = self.parser(self.ent, ast.fetch.This('this'))
+        self.assertSetEqual(set(q(self.graph)), {
+            (rdflib.URIRef('http://example.com/entity#1234'), rdflib.URIRef('http://example.com/entity#1234')),
+            (rdflib.URIRef('http://example.com/entity#4321'), rdflib.URIRef('http://example.com/entity#4321')),
+            })
+
+
+## main ##
+
+if __name__ == '__main__':
+    unittest.main()
+
+## EOF ##
diff --git a/test/triple_store/sparql/test_parse_filter.py b/test/triple_store/sparql/test_parse_filter.py
new file mode 100644
index 0000000..a45f2ef
--- /dev/null
+++ b/test/triple_store/sparql/test_parse_filter.py
@@ -0,0 +1,777 @@
+
+# imports
+import rdflib
+import unittest
+
+# bsie imports
+from bsfs import schema as bsc
+from bsfs.namespace import ns
+from bsfs.query import ast
+from bsfs.utils import errors
+
+# objects to test
+from bsfs.triple_store.sparql.parse_filter import Filter
+
+
+## code ##
+
+ns.bse = ns.bsfs.Entity()
+
+class TestParseFilter(unittest.TestCase):
+    def setUp(self):
+        # schema
+        self.schema = bsc.from_string('''
+            prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
+            prefix xsd: <http://www.w3.org/2001/XMLSchema#>
+
+            prefix bsfs: <https://schema.bsfs.io/core/>
+            prefix bse: <https://schema.bsfs.io/core/Entity#>
+            prefix bsl: <https://schema.bsfs.io/core/Literal/>
+            prefix bsd: <https://schema.bsfs.io/core/distance#>
+            prefix bsa: <https://schema.bsfs.io/core/Literal/Array/>
+
+            bsl:Array rdfs:subClassOf bsfs:Literal .
+            bsa:Feature rdfs:subClassOf bsl:Array .
+            bsl:Number rdfs:subClassOf bsfs:Literal .
+
+            bsfs:Entity rdfs:subClassOf bsfs:Node .
+            bsfs:Image rdfs:subClassOf bsfs:Entity .
+            bsfs:Tag rdfs:subClassOf bsfs:Node .
+
+            xsd:string rdfs:subClassOf bsfs:Literal .
+            xsd:integer rdfs:subClassOf bsl:Number .
+            bsfs:URI rdfs:subClassOf bsfs:Literal .
+
+            bsfs:Colors rdfs:subClassOf bsa:Feature ;
+                bsfs:dimension "4"^^xsd:integer ;
+                bsfs:dtype xsd:integer ;
+                bsfs:distance bsd:euclidean .
+
+            bse:colors rdfs:subClassOf bsfs:Predicate ;
+                rdfs:domain bsfs:Entity ;
+                rdfs:range bsfs:Colors .
+
+            bse:comment rdfs:subClassOf bsfs:Predicate ;
+                rdfs:domain bsfs:Node ;
+                rdfs:range xsd:string ;
+                bsfs:unique "false"^^xsd:boolean .
+
+            bse:filesize rdfs:subClassOf bsfs:Predicate ;
+                rdfs:domain bsfs:Entity ;
+                rdfs:range xsd:integer ;
+                bsfs:unique "true"^^xsd:boolean .
+
+            bse:buddy rdfs:subClassOf bsfs:Predicate ;
+                rdfs:domain bsfs:Entity ;
+                rdfs:range bsfs:Node ;
+                bsfs:unique "false"^^xsd:boolean .
+
+            bse:tag rdfs:subClassOf bsfs:Predicate ;
+                rdfs:domain bsfs:Entity ;
+                rdfs:range bsfs:Tag ;
+                bsfs:unique "false"^^xsd:boolean .
+
+            bse:representative rdfs:subClassOf bsfs:Predicate ;
+                rdfs:domain bsfs:Tag ;
+                rdfs:range bsfs:Image ;
+                bsfs:unique "false"^^xsd:boolean .
+
+            bse:iso rdfs:subClassOf bsfs:Predicate ;
+                rdfs:domain bsfs:Image ;
+                rdfs:range xsd:integer ;
+                bsfs:unique "true"^^xsd:boolean .
+
+            ''')
+
+        # graph to test queries
+        self.graph = rdflib.Graph()
+        # schema hierarchies
+        self.graph.add((rdflib.URIRef('https://schema.bsfs.io/core/Entity'), rdflib.RDFS.subClassOf, rdflib.URIRef('https://schema.bsfs.io/core/Node')))
+        self.graph.add((rdflib.URIRef('https://schema.bsfs.io/core/Image'), rdflib.RDFS.subClassOf, rdflib.URIRef('https://schema.bsfs.io/core/Entity')))
+        self.graph.add((rdflib.URIRef('https://schema.bsfs.io/core/Tag'), rdflib.RDFS.subClassOf, rdflib.URIRef('https://schema.bsfs.io/core/Node')))
+        # entities
+        self.graph.add((rdflib.URIRef('http://example.com/entity#1234'), rdflib.RDF.type, rdflib.URIRef('https://schema.bsfs.io/core/Entity')))
+        self.graph.add((rdflib.URIRef('http://example.com/entity#4321'), rdflib.RDF.type, rdflib.URIRef('https://schema.bsfs.io/core/Entity')))
+        # tags
+        self.graph.add((rdflib.URIRef('http://example.com/tag#1234'), rdflib.RDF.type, rdflib.URIRef('https://schema.bsfs.io/core/Tag')))
+        self.graph.add((rdflib.URIRef('http://example.com/tag#4321'), rdflib.RDF.type, rdflib.URIRef('https://schema.bsfs.io/core/Tag')))
+        # images
+        self.graph.add((rdflib.URIRef('http://example.com/image#1234'), rdflib.RDF.type, rdflib.URIRef('https://schema.bsfs.io/core/Image')))
+        self.graph.add((rdflib.URIRef('http://example.com/image#4321'), rdflib.RDF.type, rdflib.URIRef('https://schema.bsfs.io/core/Image')))
+        # node comments
+        self.graph.add((rdflib.URIRef('http://example.com/entity#1234'), rdflib.URIRef(ns.bse.comment), rdflib.Literal('Me, Myself, and I', datatype=rdflib.XSD.string)))
+        self.graph.add((rdflib.URIRef('http://example.com/entity#1234'), rdflib.URIRef(ns.bse.comment), rdflib.Literal('hello world', datatype=rdflib.XSD.string)))
+        self.graph.add((rdflib.URIRef('http://example.com/entity#4321'), rdflib.URIRef(ns.bse.comment), rdflib.Literal('hello world', datatype=rdflib.XSD.string)))
+        self.graph.add((rdflib.URIRef('http://example.com/image#1234'), rdflib.URIRef(ns.bse.comment), rdflib.Literal('Me, Myself, and I', datatype=rdflib.XSD.string)))
+        self.graph.add((rdflib.URIRef('http://example.com/tag#1234'), rdflib.URIRef(ns.bse.comment), rdflib.Literal('Me, Myself, and I', datatype=rdflib.XSD.string)))
+        self.graph.add((rdflib.URIRef('http://example.com/tag#4321'), rdflib.URIRef(ns.bse.comment), rdflib.Literal('4321', datatype=rdflib.XSD.string)))
+        # entity filesizes
+        self.graph.add((rdflib.URIRef('http://example.com/entity#1234'), rdflib.URIRef(ns.bse.filesize), rdflib.Literal(1234, datatype=rdflib.XSD.integer)))
+        self.graph.add((rdflib.URIRef('http://example.com/entity#4321'), rdflib.URIRef(ns.bse.filesize), rdflib.Literal(4321, datatype=rdflib.XSD.integer)))
+        self.graph.add((rdflib.URIRef('http://example.com/image#1234'), rdflib.URIRef(ns.bse.filesize), rdflib.Literal(1234, datatype=rdflib.XSD.integer)))
+        self.graph.add((rdflib.URIRef('http://example.com/image#4321'), rdflib.URIRef(ns.bse.filesize), rdflib.Literal(4321, datatype=rdflib.XSD.integer)))
+        # entity tags
+        self.graph.add((rdflib.URIRef('http://example.com/entity#1234'), rdflib.URIRef(ns.bse.tag), rdflib.URIRef('http://example.com/tag#1234')))
+        self.graph.add((rdflib.URIRef('http://example.com/entity#4321'), rdflib.URIRef(ns.bse.tag), rdflib.URIRef('http://example.com/tag#4321')))
+        self.graph.add((rdflib.URIRef('http://example.com/image#1234'), rdflib.URIRef(ns.bse.tag), rdflib.URIRef('http://example.com/tag#1234')))
+        # tag representatives
+        self.graph.add((rdflib.URIRef('http://example.com/tag#1234'), rdflib.URIRef(ns.bse.representative), rdflib.URIRef('http://example.com/image#1234')))
+        self.graph.add((rdflib.URIRef('http://example.com/tag#4321'), rdflib.URIRef(ns.bse.representative), rdflib.URIRef('http://example.com/image#4321')))
+        # entity buddies
+        self.graph.add((rdflib.URIRef('http://example.com/entity#1234'), rdflib.URIRef(ns.bse.buddy), rdflib.URIRef('http://example.com/image#1234')))
+        self.graph.add((rdflib.URIRef('http://example.com/entity#4321'), rdflib.URIRef(ns.bse.buddy), rdflib.URIRef('http://example.com/image#4321')))
+        # image iso
+        self.graph.add((rdflib.URIRef('http://example.com/image#1234'), rdflib.URIRef(ns.bse.iso), rdflib.Literal(1234, datatype=rdflib.XSD.integer)))
+        self.graph.add((rdflib.URIRef('http://example.com/image#4321'), rdflib.URIRef(ns.bse.iso), rdflib.Literal(4321, datatype=rdflib.XSD.integer)))
+        # color features
+        self.graph.add((rdflib.URIRef('http://example.com/entity#1234'), rdflib.URIRef(ns.bse.colors), rdflib.Literal([1,2,3,4], datatype=rdflib.URIRef(ns.bsfs.Colors))))
+        self.graph.add((rdflib.URIRef('http://example.com/entity#4321'), rdflib.URIRef(ns.bse.colors), rdflib.Literal([4,3,2,1], datatype=rdflib.URIRef(ns.bsfs.Colors))))
+        self.graph.add((rdflib.URIRef('http://example.com/image#1234'), rdflib.URIRef(ns.bse.colors), rdflib.Literal([3,4,2,1], datatype=rdflib.URIRef(ns.bsfs.Colors))))
+
+        # parser instance
+        self.parser = Filter(self.graph, self.schema)
+
+
+    def test_routing(self):
+        self.assertRaises(errors.BackendError, self.parser._parse_filter_expression, '1234', None, '')
+        self.assertRaises(errors.BackendError, self.parser._parse_predicate_expression, '1234', None)
+
+    def test_call(self):
+        # NOTE: The individual ast components are considered in the respective tests. Here, we test __call__ specifics.
+
+        # __call__ requires a valid root type
+        self.assertRaises(errors.BackendError, self.parser, self.schema.literal(ns.bsfs.Literal), None)
+        self.assertRaises(errors.ConsistencyError, self.parser, self.schema.node(ns.bsfs.Node).child(ns.bsfs.Invalid), None)
+        # __call__ requires a parseable root
+        self.assertRaises(errors.BackendError, self.parser, self.schema.node(ns.bsfs.Entity), ast.filter.FilterExpression())
+        # __call__ returns an executable query
+        q = self.parser(self.schema.node(ns.bsfs.Entity),
+            ast.filter.Or(
+                ast.filter.Is('http://example.com/entity#1234'),
+                ast.filter.Is('http://example.com/entity#5678')))
+        self.assertSetEqual({str(guid) for guid, in q(self.graph)}, {'http://example.com/entity#1234'})
+        # root is optional
+        q = self.parser(self.schema.node(ns.bsfs.Entity))
+        self.assertSetEqual({str(guid) for guid, in q(self.graph)},
+            {'http://example.com/entity#1234', 'http://example.com/entity#4321', 'http://example.com/image#1234', 'http://example.com/image#4321'})
+        q = self.parser(self.schema.node(ns.bsfs.Tag))
+        self.assertSetEqual({str(guid) for guid, in q(self.graph)},
+            {'http://example.com/tag#1234', 'http://example.com/tag#4321'})
+
+
+    def test_is(self):
+        # _is requires a node
+        self.assertRaises(errors.BackendError, self.parser._is, self.schema.literal(ns.bsfs.Literal), ast.filter.Is('http://example.com/entity#1234'), '?ent')
+        # _is requires a serializable guid
+        self.assertRaises(ValueError, self.parser._is, self.schema.node(ns.bsfs.Entity), ast.filter.Is('http://example.com/entity#foo and bar'), '?ent')
+        # a single Is statement
+        q = self.parser(self.schema.node(ns.bsfs.Entity), ast.filter.Is('http://example.com/entity#1234'))
+        self.assertSetEqual({str(guid) for guid, in q(self.graph)},
+            {'http://example.com/entity#1234'})
+        # an aggregate of Is statements
+        q = self.parser(self.schema.node(ns.bsfs.Entity),
+            ast.filter.Or(
+                ast.filter.Is('http://example.com/entity#1234'),
+                ast.filter.Is('http://example.com/entity#4321'),
+                ))
+        self.assertSetEqual({str(guid) for guid, in q(self.graph)},
+            {'http://example.com/entity#1234', 'http://example.com/entity#4321'})
+        # combined with other filters
+        q = self.parser(self.schema.node(ns.bsfs.Entity),
+            ast.filter.And(
+                ast.filter.Or(
+                    ast.filter.Is('http://example.com/entity#1234'),
+                    ast.filter.Is('http://example.com/entity#4321'),
+                    ),
+                ast.filter.Any(ns.bse.comment,
+                    ast.filter.Equals('Me, Myself, and I')
+                    ),
+                ))
+        self.assertSetEqual({str(guid) for guid, in q(self.graph)},
+            {'http://example.com/entity#1234'})
+        # as argument of Any/All
+        q = self.parser(self.schema.node(ns.bsfs.Entity),
+            ast.filter.Any(ns.bse.tag, ast.filter.Is('http://example.com/tag#1234')))
+        self.assertSetEqual({str(guid) for guid, in q(self.graph)},
+            {'http://example.com/entity#1234', 'http://example.com/image#1234'})
+
+
+    def test_equals(self):
+        # _equals requires a literal
+        self.assertRaises(errors.BackendError, self.parser._equals, self.schema.node(ns.bsfs.Entity), ast.filter.Equals('hello world'), '?ent')
+        # a single Equals statement
+        q = self.parser(self.schema.node(ns.bsfs.Entity), ast.filter.Any(ns.bse.comment, ast.filter.Equals('hello world')))
+        self.assertSetEqual({str(guid) for guid, in q(self.graph)},
+            {'http://example.com/entity#1234', 'http://example.com/entity#4321'})
+        # a single Equals statement that includes subtypes
+        q = self.parser(self.schema.node(ns.bsfs.Entity), ast.filter.Any(ns.bse.comment, ast.filter.Equals('Me, Myself, and I')))
+        self.assertSetEqual({str(guid) for guid, in q(self.graph)},
+            {'http://example.com/entity#1234', 'http://example.com/image#1234'})
+        # an Equals statement on an integer
+        q = self.parser(self.schema.node(ns.bsfs.Entity), ast.filter.Any(ns.bse.filesize, ast.filter.Equals(4321)))
+        self.assertSetEqual({str(guid) for guid, in q(self.graph)},
+            {'http://example.com/entity#4321', 'http://example.com/image#4321'})
+
+
+    def test_substring(self):
+        # _substring requires a literal
+        self.assertRaises(errors.BackendError, self.parser._substring, self.schema.node(ns.bsfs.Entity), ast.filter.Substring('hello world'), '?ent')
+        # a single Substring statement
+        q = self.parser(self.schema.node(ns.bsfs.Entity), ast.filter.Any(ns.bse.comment, ast.filter.Substring('hello')))
+        self.assertSetEqual({str(guid) for guid, in q(self.graph)},
+            {'http://example.com/entity#1234', 'http://example.com/entity#4321'})
+        q = self.parser(self.schema.node(ns.bsfs.Entity), ast.filter.Any(ns.bse.comment, ast.filter.Substring('lo wo')))
+        self.assertSetEqual({str(guid) for guid, in q(self.graph)},
+            {'http://example.com/entity#1234', 'http://example.com/entity#4321'})
+        # a single Substring statement that includes subtypes
+        q = self.parser(self.schema.node(ns.bsfs.Entity), ast.filter.Any(ns.bse.comment, ast.filter.Substring('Myself')))
+        self.assertSetEqual({str(guid) for guid, in q(self.graph)},
+            {'http://example.com/entity#1234', 'http://example.com/image#1234'})
+        # an Substring statement on an integer
+        q = self.parser(self.schema.node(ns.bsfs.Entity), ast.filter.Any(ns.bse.filesize, ast.filter.Substring('32')))
+        self.assertSetEqual({str(guid) for guid, in q(self.graph)},
+            {'http://example.com/entity#4321', 'http://example.com/image#4321'})
+
+
+    def test_starts_with(self):
+        # _starts_with requires a literal
+        self.assertRaises(errors.BackendError, self.parser._starts_with, self.schema.node(ns.bsfs.Entity), ast.filter.StartsWith('hello world'), '?ent')
+        # a single StartsWith statement
+        q = self.parser(self.schema.node(ns.bsfs.Entity), ast.filter.Any(ns.bse.comment, ast.filter.StartsWith('hello')))
+        self.assertSetEqual({str(guid) for guid, in q(self.graph)},
+            {'http://example.com/entity#1234', 'http://example.com/entity#4321'})
+        # a single StartsWith statement that includes subtypes
+        q = self.parser(self.schema.node(ns.bsfs.Entity), ast.filter.Any(ns.bse.comment, ast.filter.StartsWith('Me, Mys')))
+        self.assertSetEqual({str(guid) for guid, in q(self.graph)},
+            {'http://example.com/entity#1234', 'http://example.com/image#1234'})
+        # an StartsWith statement on an integer
+        q = self.parser(self.schema.node(ns.bsfs.Entity), ast.filter.Any(ns.bse.filesize, ast.filter.StartsWith(432)))
+        self.assertSetEqual({str(guid) for guid, in q(self.graph)},
+            {'http://example.com/entity#4321', 'http://example.com/image#4321'})
+
+
+    def test_ends_with(self):
+        # _ends_with requires a literal
+        self.assertRaises(errors.BackendError, self.parser._ends_with, self.schema.node(ns.bsfs.Entity), ast.filter.EndsWith('hello world'), '?ent')
+        # a single EndsWith statement
+        q = self.parser(self.schema.node(ns.bsfs.Entity), ast.filter.Any(ns.bse.comment, ast.filter.EndsWith('orld')))
+        self.assertSetEqual({str(guid) for guid, in q(self.graph)},
+            {'http://example.com/entity#1234', 'http://example.com/entity#4321'})
+        # a single EndsWith statement that includes subtypes
+        q = self.parser(self.schema.node(ns.bsfs.Entity), ast.filter.Any(ns.bse.comment, ast.filter.EndsWith('and I')))
+        self.assertSetEqual({str(guid) for guid, in q(self.graph)},
+            {'http://example.com/entity#1234', 'http://example.com/image#1234'})
+        # an EndsWith statement on an integer
+        q = self.parser(self.schema.node(ns.bsfs.Entity), ast.filter.Any(ns.bse.filesize, ast.filter.EndsWith(321)))
+        self.assertSetEqual({str(guid) for guid, in q(self.graph)},
+            {'http://example.com/entity#4321', 'http://example.com/image#4321'})
+
+
+    def test_less_than(self):
+        # _less_than requires a literal
+        self.assertRaises(errors.BackendError, self.parser._less_than, self.schema.node(ns.bsfs.Entity), ast.filter.LessThan(2000), '?ent')
+        # a single LessThan statement
+        q = self.parser(self.schema.node(ns.bsfs.Image), ast.filter.Any(ns.bse.iso, ast.filter.LessThan(2000)))
+        self.assertSetEqual({str(guid) for guid, in q(self.graph)},
+            {'http://example.com/image#1234'})
+        # _less_than respects boundary
+        q = self.parser(self.schema.node(ns.bsfs.Image), ast.filter.Any(ns.bse.iso, ast.filter.LessThan(1234, strict=True)))
+        self.assertSetEqual({str(guid) for guid, in q(self.graph)}, set())
+        q = self.parser(self.schema.node(ns.bsfs.Image), ast.filter.Any(ns.bse.iso, ast.filter.LessThan(1234, strict=False)))
+        self.assertSetEqual({str(guid) for guid, in q(self.graph)},
+            {'http://example.com/image#1234'})
+        # a single LessThan statement that includes subtypes
+        q = self.parser(self.schema.node(ns.bsfs.Entity), ast.filter.Any(ns.bse.filesize, ast.filter.LessThan(2000)))
+        self.assertSetEqual({str(guid) for guid, in q(self.graph)},
+            {'http://example.com/entity#1234', 'http://example.com/image#1234'})
+        # an LessThan statement on a string
+        # always negative; note that http://example.com/tag#4321 is also not returned although its comment is a pure number
+        q = self.parser(self.schema.node(ns.bsfs.Entity), ast.filter.Any(ns.bse.comment, ast.filter.LessThan(10_000)))
+        self.assertSetEqual({str(guid) for guid, in q(self.graph)}, set())
+
+
+    def test_greater_than(self):
+        # _greater_than requires a literal
+        self.assertRaises(errors.BackendError, self.parser._greater_than, self.schema.node(ns.bsfs.Entity), ast.filter.GreaterThan(2000), '?ent')
+        # a single GreaterThan statement
+        q = self.parser(self.schema.node(ns.bsfs.Image), ast.filter.Any(ns.bse.iso, ast.filter.GreaterThan(2000)))
+        self.assertSetEqual({str(guid) for guid, in q(self.graph)},
+            {'http://example.com/image#4321'})
+        # _greater_than respects boundary
+        q = self.parser(self.schema.node(ns.bsfs.Image), ast.filter.Any(ns.bse.iso, ast.filter.GreaterThan(4321, strict=True)))
+        self.assertSetEqual({str(guid) for guid, in q(self.graph)}, set())
+        q = self.parser(self.schema.node(ns.bsfs.Image), ast.filter.Any(ns.bse.iso, ast.filter.GreaterThan(4321, strict=False)))
+        self.assertSetEqual({str(guid) for guid, in q(self.graph)},
+            {'http://example.com/image#4321'})
+        # a single GreaterThan statement that includes subtypes
+        q = self.parser(self.schema.node(ns.bsfs.Entity), ast.filter.Any(ns.bse.filesize, ast.filter.GreaterThan(2000)))
+        self.assertSetEqual({str(guid) for guid, in q(self.graph)},
+            {'http://example.com/entity#4321', 'http://example.com/image#4321'})
+        # an GreaterThan statement on a string
+        # always positive
+        q = self.parser(self.schema.node(ns.bsfs.Entity), ast.filter.Any(ns.bse.comment, ast.filter.GreaterThan(0)))
+        self.assertSetEqual({str(guid) for guid, in q(self.graph)},
+            {'http://example.com/entity#1234', 'http://example.com/entity#4321', 'http://example.com/image#1234'})
+
+
+    def test_and(self):
+        # And childs have to match the node type
+        self.assertRaises(errors.BackendError, self.parser,
+            self.schema.node(ns.bsfs.Entity),
+            ast.filter.And(
+                ast.filter.StartsWith('hello'),
+                ast.filter.EndsWith('world'),
+                ))
+        # no child produces an empty query
+        self.assertEqual(self.parser._and(
+            self.schema.node(ns.bsfs.Entity),
+            ast.filter.And(), '?ent'), '')
+        # And can mix different conditions
+        q = self.parser(self.schema.node(ns.bsfs.Entity),
+            ast.filter.And(
+                ast.filter.Is('http://example.com/entity#1234'),
+                ast.filter.Any(ns.bse.filesize, ast.filter.Equals(1234)),
+                ast.filter.Any(ns.bse.comment, ast.filter.Equals('Me, Myself, and I')),
+                ))
+        self.assertSetEqual({str(guid) for guid, in q(self.graph)},
+            {'http://example.com/entity#1234'})
+        # all conditions have to match
+        q = self.parser(self.schema.node(ns.bsfs.Entity),
+            ast.filter.And(
+                ast.filter.Is('http://example.com/entity#4321'),
+                ast.filter.Any(ns.bse.filesize, ast.filter.Equals(1234)),
+                ast.filter.Any(ns.bse.comment, ast.filter.Equals('Me, Myself, and I')),
+                ))
+        self.assertSetEqual({str(guid) for guid, in q(self.graph)}, set())
+        q = self.parser(self.schema.node(ns.bsfs.Entity),
+            ast.filter.And(
+                ast.filter.Is('http://example.com/entity#1234'),
+                ast.filter.Any(ns.bse.filesize, ast.filter.Equals(4321)),
+                ast.filter.Any(ns.bse.comment, ast.filter.Equals('Me, Myself, and I')),
+                ))
+        self.assertSetEqual({str(guid) for guid, in q(self.graph)}, set())
+        q = self.parser(self.schema.node(ns.bsfs.Entity),
+            ast.filter.And(
+                ast.filter.Is('http://example.com/entity#1234'),
+                ast.filter.Any(ns.bse.filesize, ast.filter.Equals(1234)),
+                ast.filter.Any(ns.bse.comment, ast.filter.Equals('foobar')),
+                ))
+        self.assertSetEqual({str(guid) for guid, in q(self.graph)}, set())
+        # And can be nested
+        q = self.parser(self.schema.node(ns.bsfs.Entity),
+            ast.filter.And(
+                ast.filter.Is('http://example.com/entity#1234'),
+                ast.filter.And(
+                    ast.filter.Any(ns.bse.filesize, ast.filter.Equals(1234)),
+                    ast.filter.Any(ns.bse.comment, ast.filter.Equals('Me, Myself, and I')),
+                    ),
+                ))
+        self.assertSetEqual({str(guid) for guid, in q(self.graph)},
+            {'http://example.com/entity#1234'})
+
+
+    def test_or(self):
+        # Or childs have to match the node type
+        self.assertRaises(errors.BackendError, self.parser,
+            self.schema.node(ns.bsfs.Entity),
+            ast.filter.Or(
+                ast.filter.StartsWith('hello'),
+                ast.filter.EndsWith('world'),
+                ))
+        # no child produces an empty query
+        self.assertEqual(self.parser._and(
+            self.schema.node(ns.bsfs.Entity),
+            ast.filter.Or(), '?ent'), '')
+        # Or can mix different conditions
+        q = self.parser(self.schema.node(ns.bsfs.Entity),
+            ast.filter.Or(
+                ast.filter.Is('http://example.com/entity#1234'),
+                ast.filter.Any(ns.bse.filesize, ast.filter.Equals(4321)),
+                ast.filter.Any(ns.bse.comment, ast.filter.Equals('Me, Myself, and I')),
+                ))
+        self.assertSetEqual({str(guid) for guid, in q(self.graph)},
+                {'http://example.com/entity#1234', 'http://example.com/image#1234', 'http://example.com/entity#4321', 'http://example.com/image#4321'})
+        # at least one condition has to match
+        q = self.parser(self.schema.node(ns.bsfs.Entity),
+            ast.filter.Or(
+                ast.filter.Is('http://example.com/entity#5678'),
+                ast.filter.Any(ns.bse.filesize, ast.filter.Equals(8765)),
+                ast.filter.Any(ns.bse.comment, ast.filter.Equals('foobar')),
+                ))
+        self.assertSetEqual({str(guid) for guid, in q(self.graph)}, set())
+        q = self.parser(self.schema.node(ns.bsfs.Entity),
+            ast.filter.Or(
+                ast.filter.Is('http://example.com/entity#1234'),
+                ast.filter.Any(ns.bse.filesize, ast.filter.Equals(8765)),
+                ast.filter.Any(ns.bse.comment, ast.filter.Equals('foobar')),
+                ))
+        self.assertSetEqual({str(guid) for guid, in q(self.graph)},
+            {'http://example.com/entity#1234'})
+        q = self.parser(self.schema.node(ns.bsfs.Entity),
+            ast.filter.Or(
+                ast.filter.Is('http://example.com/entity#5678'),
+                ast.filter.Any(ns.bse.filesize, ast.filter.Equals(4321)),
+                ast.filter.Any(ns.bse.comment, ast.filter.Equals('foobar')),
+                ))
+        self.assertSetEqual({str(guid) for guid, in q(self.graph)},
+            {'http://example.com/entity#4321', 'http://example.com/image#4321'})
+        q = self.parser(self.schema.node(ns.bsfs.Entity),
+            ast.filter.Or(
+                ast.filter.Is('http://example.com/entity#5678'),
+                ast.filter.Any(ns.bse.filesize, ast.filter.Equals(8765)),
+                ast.filter.Any(ns.bse.comment, ast.filter.Equals('Me, Myself, and I')),
+                ))
+        self.assertSetEqual({str(guid) for guid, in q(self.graph)},
+            {'http://example.com/entity#1234', 'http://example.com/image#1234'})
+        # Or can be nested
+        q = self.parser(self.schema.node(ns.bsfs.Entity),
+            ast.filter.Or(
+                ast.filter.Is('http://example.com/entity#1234'),
+                ast.filter.Or(
+                    ast.filter.Any(ns.bse.filesize, ast.filter.Equals(4321)),
+                    ast.filter.Any(ns.bse.comment, ast.filter.Equals('Me, Myself, and I')),
+                    ),
+                ))
+        self.assertSetEqual({str(guid) for guid, in q(self.graph)},
+                {'http://example.com/entity#1234', 'http://example.com/image#1234', 'http://example.com/entity#4321', 'http://example.com/image#4321'})
+
+
+
+    def test_any(self):
+        # _any requires a node
+        self.assertRaises(errors.BackendError, self.parser._any,
+            self.schema.literal(ns.bsfs.Literal),
+            ast.filter.Any(ns.bse.filesize, ast.filter.Equals(1234)), '?ent')
+        # node type must match predicate's domain
+        self.assertRaises(errors.ConsistencyError, self.parser._any,
+            self.schema.node(ns.bsfs.Tag),
+            ast.filter.Any(ns.bse.filesize, ast.filter.Equals(1234)), '?ent')
+        # predicate must be valid
+        self.assertRaises(errors.ConsistencyError, self.parser._any,
+            self.schema.node(ns.bsfs.Entity),
+            ast.filter.Any(ns.bse.invalid, ast.filter.Equals(1234)), '?ent')
+        # _any returns a valid query
+        q = self.parser(self.schema.node(ns.bsfs.Entity),
+            ast.filter.Any(ns.bse.filesize, ast.filter.Equals(1234)))
+        self.assertSetEqual({str(guid) for guid, in q(self.graph)},
+            {'http://example.com/entity#1234', 'http://example.com/image#1234'})
+        # _any can be nested
+        q = self.parser(self.schema.node(ns.bsfs.Entity),
+            ast.filter.Any(ns.bse.tag,
+                ast.filter.Any(ns.bse.representative,
+                    ast.filter.Is('http://example.com/image#1234'))))
+        self.assertSetEqual({str(guid) for guid, in q(self.graph)},
+            {'http://example.com/entity#1234', 'http://example.com/image#1234'})
+
+
+    def test_all(self):
+        # All requires a Node
+        self.assertRaises(errors.BackendError, self.parser._all, self.schema.literal(ns.bsfs.Literal), None, '')
+        # All Nodes
+        q = self.parser(self.schema.node(ns.bsfs.Entity),
+            ast.filter.All(ns.bse.tag, ast.filter.Is('http://example.com/tag#1234')))
+        self.assertSetEqual({str(guid) for guid, in q(self.graph)},
+            {'http://example.com/entity#1234', 'http://example.com/image#1234'})
+        # All values
+        q = self.parser(self.schema.node(ns.bsfs.Entity),
+            ast.filter.All(ns.bse.comment, ast.filter.Equals('hello world')))
+        self.assertSetEqual({str(guid) for guid, in q(self.graph)},
+            {'http://example.com/entity#4321'})
+        # All on value within Or branch
+        # entity#1234 is selected because all of its comments are in ("hello world", "Me, Myself, and I")
+        q = self.parser(self.schema.node(ns.bsfs.Entity),
+            ast.filter.All(ns.bse.comment, ast.filter.Or(
+                ast.filter.Equals('hello world'),
+                ast.filter.Equals('Me, Myself, and I'))))
+        self.assertSetEqual({str(guid) for guid, in q(self.graph)},
+            {'http://example.com/entity#1234', 'http://example.com/entity#4321', 'http://example.com/image#1234'})
+        # All requires at least one predicate/value
+        q = self.parser(self.schema.node(ns.bsfs.Entity),
+            ast.filter.All(ns.bse.comment, ast.filter.Equals('Me, Myself, and I')))
+        self.assertSetEqual({str(guid) for guid, in q(self.graph)},
+            {'http://example.com/image#1234'})
+        # All within a statement
+        q = self.parser(self.schema.node(ns.bsfs.Entity),
+            ast.filter.And(
+                ast.filter.All(ns.bse.tag, ast.filter.Is('http://example.com/tag#1234')), # entity#1234, image#1234
+                ast.filter.All(ns.bse.comment, ast.filter.Or( # entity#1234, entity#4321, image#1234
+                    ast.filter.Equals('hello world'),
+                    ast.filter.Equals('Me, Myself, and I'),
+                    ))
+                )
+            )
+        self.assertSetEqual({str(guid) for guid, in q(self.graph)},
+            {'http://example.com/entity#1234', 'http://example.com/image#1234'})
+        # All with reversed Predicate
+        q = self.parser(self.schema.node(ns.bsfs.Tag),
+                ast.filter.All(ast.filter.Predicate(ns.bse.tag, reverse=True), ast.filter.Is('http://example.com/entity#4321')))
+        self.assertSetEqual({str(guid) for guid, in q(self.graph)},
+            {'http://example.com/tag#4321'})
+        # All with multiple predicates
+        q = self.parser(self.schema.node(ns.bsfs.Entity),
+                ast.filter.All(ast.filter.OneOf(ns.bse.tag, ns.bse.buddy), # entity#1234 (tag:tag#1234), entity#1234 (buddy:image#1234), image#1234(tag:tag#1234)
+                    ast.filter.Any(ns.bse.comment, ast.filter.Equals('Me, Myself, and I')))) # entity#1234, image#1234, tag#1234
+        self.assertSetEqual({str(guid) for guid, in q(self.graph)},
+            {'http://example.com/entity#1234', 'http://example.com/image#1234'})
+
+
+
+    def test_not(self):
+        # Not applies on conditions
+        q = self.parser(self.schema.node(ns.bsfs.Entity),
+            ast.filter.Not(ast.filter.Is('http://example.com/entity#1234')))
+        self.assertSetEqual({str(guid) for guid, in q(self.graph)},
+            {'http://example.com/image#1234', 'http://example.com/entity#4321', 'http://example.com/image#4321'})
+        # Not applies on conditions within branches
+        q = self.parser(self.schema.node(ns.bsfs.Entity),
+            ast.filter.Any(ns.bse.comment, ast.filter.Not(ast.filter.Equals('Me, Myself, and I'))))
+        self.assertSetEqual({str(guid) for guid, in q(self.graph)},
+            {'http://example.com/entity#1234', 'http://example.com/entity#4321'})
+        # Not applies on branches
+        q = self.parser(self.schema.node(ns.bsfs.Entity),
+            ast.filter.Not(ast.filter.Any(ns.bse.comment, ast.filter.Equals('Me, Myself, and I'))))
+        self.assertSetEqual({str(guid) for guid, in q(self.graph)},
+            {'http://example.com/entity#4321', 'http://example.com/image#4321'})
+        # Double Not cancel each other
+        q = self.parser(self.schema.node(ns.bsfs.Entity),
+            ast.filter.Not(ast.filter.Not(ast.filter.Is('http://example.com/entity#1234'))))
+        self.assertSetEqual({str(guid) for guid, in q(self.graph)},
+            {'http://example.com/entity#1234'})
+        # Not works within aggregation (and)
+        q = self.parser(self.schema.node(ns.bsfs.Entity),
+            ast.filter.And(
+                ast.filter.Not(ast.filter.Is('http://example.com/entity#1234')),
+                ast.filter.Any(ns.bse.comment, ast.filter.Equals('hello world')),
+                ))
+        self.assertSetEqual({str(guid) for guid, in q(self.graph)},
+            {'http://example.com/entity#4321'})
+        # Not works within aggregation (or)
+        q = self.parser(self.schema.node(ns.bsfs.Entity),
+            ast.filter.Or(
+                ast.filter.Not(ast.filter.Is('http://example.com/entity#1234')),
+                ast.filter.Any(ns.bse.comment, ast.filter.Equals('Me, Myself, and I')),
+                ))
+        self.assertSetEqual({str(guid) for guid, in q(self.graph)},
+            {'http://example.com/entity#1234', 'http://example.com/entity#4321', 'http://example.com/image#1234', 'http://example.com/image#4321'})
+        # Not works outside aggregation (and)
+        q = self.parser(self.schema.node(ns.bsfs.Entity),
+            ast.filter.Not(
+                ast.filter.And(
+                    ast.filter.Is('http://example.com/entity#1234'),
+                    ast.filter.Any(ns.bse.comment, ast.filter.Equals('hello world')),
+                )))
+        self.assertSetEqual({str(guid) for guid, in q(self.graph)},
+                {'http://example.com/entity#4321', 'http://example.com/image#1234', 'http://example.com/image#4321'})
+        # Not works outside aggregation (or)
+        q = self.parser(self.schema.node(ns.bsfs.Entity),
+            ast.filter.Not(
+                ast.filter.Or(
+                    ast.filter.Is('http://example.com/entity#4321'),
+                    ast.filter.Any(ns.bse.comment, ast.filter.Equals('Me, Myself, and I')),
+                )))
+        self.assertSetEqual({str(guid) for guid, in q(self.graph)},
+            {'http://example.com/image#4321'})
+        # Not mixed with branch, aggregation, id, and value
+        q = self.parser(self.schema.node(ns.bsfs.Entity),
+            ast.filter.And(
+                ast.filter.Not( # image#1234, image#4321
+                    ast.filter.Or( # entity#4321, entity#1234
+                        ast.filter.Is('http://example.com/entity#4321'),
+                        ast.filter.Any(ns.bse.comment, ast.filter.Equals('hello world')),
+                        )
+                    ),
+                ast.filter.Any(ns.bse.comment, ast.filter.Not(ast.filter.Equals('foobar'))), # entity#1234, entity#4321, image#1234
+                ))
+        self.assertSetEqual({str(guid) for guid, in q(self.graph)},
+            {'http://example.com/image#1234'})
+
+
+    def test_has(self):
+        # Has requires Node
+        self.assertRaises(errors.BackendError, self.parser._has, self.schema.literal(ns.bsfs.Literal), None, '')
+        # Has with GreaterThan constraint
+        q = self.parser(self.schema.node(ns.bsfs.Entity),
+            ast.filter.Has(ns.bse.comment, ast.filter.GreaterThan(0)))
+        self.assertSetEqual({str(guid) for guid, in q(self.graph)},
+            {'http://example.com/entity#1234', 'http://example.com/entity#4321', 'http://example.com/image#1234'})
+        q = self.parser(self.schema.node(ns.bsfs.Entity),
+            ast.filter.Has(ns.bse.comment, ast.filter.GreaterThan(1)))
+        self.assertSetEqual({str(guid) for guid, in q(self.graph)},
+            {'http://example.com/entity#1234'})
+        # Has with Equals constraint
+        q = self.parser(self.schema.node(ns.bsfs.Entity),
+            ast.filter.Has(ns.bse.comment, 1))
+        self.assertSetEqual({str(guid) for guid, in q(self.graph)},
+            {'http://example.com/entity#4321', 'http://example.com/image#1234'})
+        # Has with LessThan constraint
+        q = self.parser(self.schema.node(ns.bsfs.Entity),
+            ast.filter.Has(ns.bse.comment, ast.filter.LessThan(2)))
+        self.assertSetEqual({str(guid) for guid, in q(self.graph)},
+            {'http://example.com/entity#4321', 'http://example.com/image#1234', 'http://example.com/image#4321'})
+        # Has with multiple constraints
+        self.graph.add((rdflib.URIRef('http://example.com/entity#1234'), rdflib.URIRef(ns.bse.comment), rdflib.Literal('extra1', datatype=rdflib.XSD.string)))
+        self.graph.add((rdflib.URIRef('http://example.com/entity#1234'), rdflib.URIRef(ns.bse.comment), rdflib.Literal('extra2', datatype=rdflib.XSD.string)))
+        self.graph.add((rdflib.URIRef('http://example.com/entity#1234'), rdflib.URIRef(ns.bse.comment), rdflib.Literal('extra3', datatype=rdflib.XSD.string)))
+        self.graph.add((rdflib.URIRef('http://example.com/entity#1234'), rdflib.URIRef(ns.bse.comment), rdflib.Literal('extra4', datatype=rdflib.XSD.string)))
+        self.graph.add((rdflib.URIRef('http://example.com/entity#1234'), rdflib.URIRef(ns.bse.comment), rdflib.Literal('extra5', datatype=rdflib.XSD.string)))
+        self.graph.add((rdflib.URIRef('http://example.com/entity#4321'), rdflib.URIRef(ns.bse.comment), rdflib.Literal('extra1', datatype=rdflib.XSD.string)))
+        self.graph.add((rdflib.URIRef('http://example.com/entity#4321'), rdflib.URIRef(ns.bse.comment), rdflib.Literal('extra2', datatype=rdflib.XSD.string)))
+        q = self.parser(self.schema.node(ns.bsfs.Entity), ast.filter.Has(ns.bse.comment,
+            ast.filter.And(ast.filter.GreaterThan(1), ast.filter.LessThan(5))))
+        self.assertSetEqual({str(guid) for guid, in q(self.graph)},
+            {'http://example.com/entity#4321'})
+        # Has with OneOf predicate
+        q = self.parser(self.schema.node(ns.bsfs.Entity), ast.filter.Has(ast.filter.OneOf(ns.bse.tag, ns.bse.buddy),
+            ast.filter.GreaterThan(1)))
+        self.assertSetEqual({str(guid) for guid, in q(self.graph)},
+            {'http://example.com/entity#1234', 'http://example.com/entity#4321'})
+        # Has with reversed predicate
+        q = self.parser(self.schema.node(ns.bsfs.Tag), ast.filter.Has(ast.filter.Predicate(ns.bse.tag, reverse=True),
+            ast.filter.GreaterThan(1)))
+        self.assertSetEqual({str(guid) for guid, in q(self.graph)},
+            {'http://example.com/tag#1234'})
+
+
+    def test_distance(self):
+        # node          colors      distance to [2,4,3,1]
+        # entity#1234   [1,2,3,4]   3.742
+        # entity#4321   [4,3,2,1]   2.449
+        # image#1234    [3,4,2,1]   1.414
+
+        # _distance expects a feature
+        self.assertRaises(errors.BackendError, self.parser._distance, self.schema.node(ns.bsfs.Entity), ast.filter.Distance([1,2,3,4], 1), '')
+        # reference must have the correct dimension
+        self.assertRaises(errors.ConsistencyError, self.parser._distance, self.schema.literal(ns.bsfs.Colors), ast.filter.Distance([1,2,3], 1), '')
+        self.assertRaises(errors.ConsistencyError, self.parser._distance, self.schema.literal(ns.bsfs.Colors), ast.filter.Distance([1,2,3,4,5], 1), '')
+        # _distance respects threshold
+        q = self.parser(self.schema.node(ns.bsfs.Entity), ast.filter.Any(ns.bse.colors, ast.filter.Distance([2,4,3,1], 4)))
+        self.assertSetEqual({str(guid) for guid, in q(self.graph)},
+            {'http://example.com/entity#1234', 'http://example.com/entity#4321', 'http://example.com/image#1234'})
+        q = self.parser(self.schema.node(ns.bsfs.Entity), ast.filter.Any(ns.bse.colors, ast.filter.Distance([2,4,3,1], 3)))
+        self.assertSetEqual({str(guid) for guid, in q(self.graph)},
+            {'http://example.com/entity#4321', 'http://example.com/image#1234'})
+        q = self.parser(self.schema.node(ns.bsfs.Entity), ast.filter.Any(ns.bse.colors, ast.filter.Distance([2,4,3,1], 2)))
+        self.assertSetEqual({str(guid) for guid, in q(self.graph)},
+            {'http://example.com/image#1234'})
+        # result set can be empty
+        q = self.parser(self.schema.node(ns.bsfs.Entity), ast.filter.Any(ns.bse.colors, ast.filter.Distance([2,4,3,1], 1)))
+        self.assertSetEqual({str(guid) for guid, in q(self.graph)}, set())
+        # _distance respects strict
+        q = self.parser(self.schema.node(ns.bsfs.Entity), ast.filter.Any(ns.bse.colors, ast.filter.Distance([1,2,3,4], 0, False)))
+        self.assertSetEqual({str(guid) for guid, in q(self.graph)},
+            {'http://example.com/entity#1234'})
+        q = self.parser(self.schema.node(ns.bsfs.Entity), ast.filter.Any(ns.bse.colors, ast.filter.Distance([1,2,3,4], 0, True)))
+        self.assertSetEqual({str(guid) for guid, in q(self.graph)}, set())
+
+    def test_one_of(self):
+        # _one_of expects a node
+        self.assertRaises(errors.BackendError, self.parser._one_of,
+            self.schema.literal(ns.bsfs.Literal),
+            ast.filter.OneOf(ast.filter.Predicate(ns.bse.filesize)))
+        # invalid predicate for node type raises an error
+        self.assertRaises(errors.ConsistencyError, self.parser._one_of,
+            self.schema.node(ns.bsfs.Node),
+            ast.filter.OneOf(ast.filter.Predicate(ns.bse.filesize)))
+        self.assertRaises(errors.ConsistencyError, self.parser,
+            self.schema.node(ns.bsfs.Tag),
+            ast.filter.Any(ast.filter.OneOf(ast.filter.Predicate(ns.bse.filesize)), ast.filter.Equals(1234)))
+        self.assertRaises(errors.BackendError, self.parser._one_of,
+            self.schema.node(ns.bsfs.Node),
+            ast.filter.OneOf(ast.filter.Predicate(ns.bsfs.Predicate)))
+        # invalid predicate combinations raise an error
+        self.assertRaises(errors.ConsistencyError, self.parser._one_of,
+            self.schema.node(ns.bsfs.Node),
+            ast.filter.OneOf(
+                ast.filter.Predicate(ns.bse.filesize),
+                ast.filter.Predicate(ns.bse.representative)))
+        # _one_of returns the URI and range
+        q = self.parser._one_of(self.schema.node(ns.bsfs.Image),
+            ast.filter.OneOf(
+                ast.filter.Predicate(ns.bse.iso),
+                ast.filter.Predicate(ns.bse.filesize)))
+        self.assertTrue(q[0] == f'<{ns.bse.iso}>|<{ns.bse.filesize}>' or q[0] == f'<{ns.bse.filesize}>|<{ns.bse.iso}>')
+        self.assertEqual(q[1], self.schema.literal(ns.xsd.integer))
+        # OneOf can be nested
+        q = self.parser._one_of(self.schema.node(ns.bsfs.Image),
+            ast.filter.OneOf(
+                ast.filter.Predicate(ns.bse.iso),
+                ast.filter.OneOf(
+                    ast.filter.Predicate(ns.bse.filesize))))
+        self.assertTrue(q[0] == f'<{ns.bse.iso}>|<{ns.bse.filesize}>' or q[0] == f'<{ns.bse.filesize}>|<{ns.bse.iso}>')
+        self.assertEqual(q[1], self.schema.literal(ns.xsd.integer))
+        # _one_of returns the most generic range
+        q = self.parser._one_of(self.schema.node(ns.bsfs.Entity),
+            ast.filter.OneOf(
+                ast.filter.Predicate(ns.bse.tag),
+                ast.filter.Predicate(ns.bse.buddy)))
+        self.assertTrue(q[0] == f'<{ns.bse.tag}>|<{ns.bse.buddy}>' or q[0] == f'<{ns.bse.buddy}>|<{ns.bse.tag}>')
+        self.assertEqual(q[1], self.schema.node(ns.bsfs.Node))
+        # domains must match the given type
+        self.assertRaises(errors.ConsistencyError, self.parser,
+            self.schema.node(ns.bsfs.Entity),
+            ast.filter.Any(ast.filter.OneOf(ns.bse.tag, ns.bse.buddy),
+                ast.filter.Any(ast.filter.OneOf(ns.bse.filesize),
+                    ast.filter.Equals(1234))))
+        # ranges must have the same type (Node/Literal)
+        self.assertRaises(errors.ConsistencyError, self.parser,
+            self.schema.node(ns.bsfs.Entity),
+            ast.filter.Any(ast.filter.OneOf(ns.bse.tag, ns.bse.filesize),
+                ast.filter.Equals(1234)))
+        # ranges must be related
+        self.assertRaises(errors.ConsistencyError, self.parser,
+            self.schema.node(ns.bsfs.Entity),
+            ast.filter.Any(ast.filter.OneOf(ns.bse.comment, ns.bse.filesize),
+                ast.filter.Equals(1234)))
+        # integration: _one_of returns a valid sparql query
+        q = self.parser(self.schema.node(ns.bsfs.Entity),
+            ast.filter.Any(ast.filter.OneOf(ns.bse.tag, ns.bse.buddy),
+                ast.filter.Any(ast.filter.OneOf(ns.bse.comment),
+                    ast.filter.Equals('Me, Myself, and I'))))
+        self.assertSetEqual({str(guid) for guid, in q(self.graph)},
+            {'http://example.com/entity#1234', 'http://example.com/image#1234'})
+
+
+    def test_predicate(self):
+        # predicate cannot be the root predicate (ns.bsfs.Predicate)
+        self.assertRaises(errors.BackendError, self.parser._predicate, self.schema.node(ns.bsfs.Node), ast.filter.Predicate(ns.bsfs.Predicate))
+        # _predicate expects a node
+        self.assertRaises(errors.BackendError, self.parser._predicate,
+            self.schema.literal(ns.bsfs.Literal),
+            ast.filter.Predicate(ns.bse.filesize))
+        # invalid predicate for node type raises an error
+        self.assertRaises(errors.ConsistencyError, self.parser._predicate,
+            self.schema.node(ns.bsfs.Node),
+            ast.filter.Predicate(ns.bse.filesize))
+        self.assertRaises(errors.ConsistencyError, self.parser,
+            self.schema.node(ns.bsfs.Tag),
+            ast.filter.Any(ast.filter.Predicate(ns.bse.filesize), ast.filter.Equals(1234)))
+        # _predicate returns the URI and range
+        self.assertEqual(self.parser._predicate(self.schema.node(ns.bsfs.Entity), ast.filter.Predicate(ns.bse.filesize)),
+            (f'<{ns.bse.filesize}>', self.schema.literal(ns.xsd.integer)))
+        self.assertEqual(self.parser._predicate(self.schema.node(ns.bsfs.Entity), ast.filter.Predicate(ns.bse.tag)),
+            (f'<{ns.bse.tag}>', self.schema.node(ns.bsfs.Tag)))
+        # _predicate respects reverse flag
+        self.assertEqual(self.parser._predicate(self.schema.node(ns.bsfs.Tag), ast.filter.Predicate(ns.bse.tag, reverse=True)),
+            ('^<' + ns.bse.tag + '>', self.schema.node(ns.bsfs.Entity)))
+        # integration: _predicate returns a valid sparql query
+        q = self.parser(self.schema.node(ns.bsfs.Entity),
+            ast.filter.Any(ns.bse.tag,
+                ast.filter.Any(ns.bse.representative,
+                    ast.filter.Any(ns.bse.filesize,
+                        ast.filter.Equals(1234)))))
+        self.assertSetEqual({str(guid) for guid, in q(self.graph)},
+            {'http://example.com/entity#1234', 'http://example.com/image#1234'})
+        q = self.parser(self.schema.node(ns.bsfs.Tag),
+            ast.filter.Any(ast.filter.Predicate(ns.bse.tag, reverse=True),
+                ast.filter.Any(ns.bse.filesize,
+                    ast.filter.LessThan(2000))))
+        self.assertSetEqual({str(guid) for guid, in q(self.graph)},
+            {'http://example.com/tag#1234'})
+
+
+## main ##
+
+if __name__ == '__main__':
+    unittest.main()
+
+## EOF ##
diff --git a/test/triple_store/test_sparql.py b/test/triple_store/sparql/test_sparql.py
index 8d98749..a7e7d37 100644
--- a/test/triple_store/test_sparql.py
+++ b/test/triple_store/sparql/test_sparql.py
@@ -1,38 +1,39 @@
-"""
 
-Part of the bsfs test suite.
-A copy of the license is provided with the project.
-Author: Matthias Baumgartner, 2022
-"""
 # imports
 import rdflib
 import unittest
 
 # bsie imports
-from bsfs import schema as _schema
+from bsfs import schema as bsc
 from bsfs.namespace import ns
+from bsfs.query import ast
 from bsfs.utils import errors, URI
 
 # objects to test
-from bsfs.triple_store.sparql import SparqlStore
+from bsfs.triple_store.sparql.sparql import SparqlStore
 
 
 ## code ##
 
+ns.bse = ns.bsfs.Entity()
+
 class TestSparqlStore(unittest.TestCase):
     def setUp(self):
-        self.schema = _schema.Schema.from_string('''
+        self.schema = bsc.from_string('''
             prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
             prefix xsd: <http://www.w3.org/2001/XMLSchema#>
 
-            prefix bsfs: <http://bsfs.ai/schema/>
-            prefix bse: <http://bsfs.ai/schema/Entity#>
+            prefix bsfs: <https://schema.bsfs.io/core/>
+            prefix bse: <https://schema.bsfs.io/core/Entity#>
+            prefix bsl: <https://schema.bsfs.io/core/Literal/>
 
             bsfs:Entity rdfs:subClassOf bsfs:Node .
             bsfs:Tag rdfs:subClassOf bsfs:Node .
             bsfs:User rdfs:subClassOf bsfs:Node .
             xsd:string rdfs:subClassOf bsfs:Literal .
-            xsd:integer rdfs:subClassOf bsfs:Literal .
+            bsl:Number rdfs:subClassOf bsfs:Literal .
+            bsl:BinaryBlob rdfs:subClassOf bsfs:Literal .
+            xsd:integer rdfs:subClassOf bsl:Number .
 
             # non-unique literal
             bse:comment rdfs:subClassOf bsfs:Predicate ;
@@ -58,7 +59,30 @@ class TestSparqlStore(unittest.TestCase):
                 rdfs:range bsfs:User ;
                 bsfs:unique "true"^^xsd:boolean .
 
+            # binary range
+            bse:asset rdfs:subClassOf bsfs:Predicate ;
+                rdfs:domain bsfs:Entity ;
+                rdfs:range bsl:BinaryBlob .
+
             ''')
+        self.schema_triples = {
+            # schema hierarchy
+            (rdflib.URIRef(ns.bsfs.Entity), rdflib.RDFS.subClassOf, rdflib.URIRef(ns.bsfs.Node)),
+            (rdflib.URIRef(ns.bsfs.Tag), rdflib.RDFS.subClassOf, rdflib.URIRef(ns.bsfs.Node)),
+            (rdflib.URIRef(ns.bsfs.User), rdflib.RDFS.subClassOf, rdflib.URIRef(ns.bsfs.Node)),
+            (rdflib.URIRef(ns.xsd.string), rdflib.RDFS.subClassOf, rdflib.URIRef(ns.bsfs.Literal)),
+            (rdflib.URIRef(ns.bsl.Array), rdflib.RDFS.subClassOf, rdflib.URIRef(ns.bsfs.Literal)),
+            (rdflib.URIRef(ns.bsl.BinaryBlob), rdflib.RDFS.subClassOf, rdflib.URIRef(ns.bsfs.Literal)),
+            (rdflib.URIRef(ns.bsl.Array.Feature), rdflib.RDFS.subClassOf, rdflib.URIRef(ns.bsl.Array)),
+            (rdflib.URIRef(ns.bsl.Number), rdflib.RDFS.subClassOf, rdflib.URIRef(ns.bsfs.Literal)),
+            (rdflib.URIRef(ns.bsl.Time), rdflib.RDFS.subClassOf, rdflib.URIRef(ns.bsfs.Literal)),
+            (rdflib.URIRef(ns.xsd.integer), rdflib.RDFS.subClassOf, rdflib.URIRef(ns.bsl.Number)),
+            (rdflib.URIRef(ns.bse.comment), rdflib.RDFS.subClassOf, rdflib.URIRef(ns.bsfs.Predicate)),
+            (rdflib.URIRef(ns.bse.filesize), rdflib.RDFS.subClassOf, rdflib.URIRef(ns.bsfs.Predicate)),
+            (rdflib.URIRef(ns.bse.tag), rdflib.RDFS.subClassOf, rdflib.URIRef(ns.bsfs.Predicate)),
+            (rdflib.URIRef(ns.bse.author), rdflib.RDFS.subClassOf, rdflib.URIRef(ns.bsfs.Predicate)),
+            (rdflib.URIRef(ns.bse.asset), rdflib.RDFS.subClassOf, rdflib.URIRef(ns.bsfs.Predicate)),
+            }
 
     def test_essentials(self):
         store = SparqlStore.Open()
@@ -77,10 +101,10 @@ class TestSparqlStore(unittest.TestCase):
     def test__has_type(self):
         # setup store
         store = SparqlStore.Open()
-        store.schema = _schema.Schema.from_string('''
+        store.schema = bsc.from_string('''
             prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
             prefix xsd: <http://www.w3.org/2001/XMLSchema#>
-            prefix bsfs: <http://bsfs.ai/schema/>
+            prefix bsfs: <https://schema.bsfs.io/core/>
 
             bsfs:Entity rdfs:subClassOf bsfs:Node .
             bsfs:Document rdfs:subClassOf bsfs:Entity .
@@ -95,7 +119,7 @@ class TestSparqlStore(unittest.TestCase):
         store.create(store.schema.node(ns.bsfs.PDF), {URI('http://example.com/me/pdf#1234')})
 
         # node_type must be in the schema
-        self.assertRaises(errors.ConsistencyError, store._has_type, URI('http://example.com/me/entity#1234'), store.schema.node(ns.bsfs.Node).get_child(ns.bsfs.invalid))
+        self.assertRaises(errors.ConsistencyError, store._has_type, URI('http://example.com/me/entity#1234'), store.schema.node(ns.bsfs.Node).child(ns.bsfs.invalid))
 
         # returns False on inexistent nodes
         self.assertFalse(store._has_type(URI('http://example.com/me/entity#4321'), store.schema.node(ns.bsfs.Entity)))
@@ -155,7 +179,7 @@ class TestSparqlStore(unittest.TestCase):
         store.set(curr.node(ns.bsfs.Entity), ent_ids, p_author,
             {URI('http://example.com/me')})
         # check instances
-        instances = {
+        instances = self.schema_triples | {
             # node instances
             (rdflib.URIRef('http://example.com/me/entity#1234'), rdflib.RDF.type, rdflib.URIRef(ns.bsfs.Entity)),
             (rdflib.URIRef('http://example.com/me/entity#4321'), rdflib.RDF.type, rdflib.URIRef(ns.bsfs.Entity)),
@@ -182,13 +206,13 @@ class TestSparqlStore(unittest.TestCase):
         self.assertSetEqual(set(store._graph), instances)
 
         # add some classes to the schema
-        curr = curr + _schema.Schema.from_string('''
+        curr = curr + bsc.from_string('''
             prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
             prefix xsd: <http://www.w3.org/2001/XMLSchema#>
-            prefix bsfs: <http://bsfs.ai/schema/>
-            prefix bse: <http://bsfs.ai/schema/Entity#>
-            prefix bst: <http://bsfs.ai/schema/Tag#>
-            prefix bsc: <http://bsfs.ai/schema/Collection#>
+            prefix bsfs: <https://schema.bsfs.io/core/>
+            prefix bse: <https://schema.bsfs.io/core/Entity#>
+            prefix bst: <https://schema.bsfs.io/core/Tag#>
+            prefix bsc: <https://schema.bsfs.io/core/Collection#>
 
             bsfs:Entity rdfs:subClassOf bsfs:Node .
             bsfs:Tag rdfs:subClassOf bsfs:Node .
@@ -228,13 +252,22 @@ class TestSparqlStore(unittest.TestCase):
         store.schema = curr
         self.assertEqual(store.schema, curr)
         # instances have not changed
-        self.assertSetEqual(set(store._graph), instances)
+        self.assertSetEqual(set(store._graph), instances | {
+            # schema hierarchy
+            (rdflib.URIRef(ns.bsfs.Collection), rdflib.RDFS.subClassOf, rdflib.URIRef(ns.bsfs.Node)),
+            (rdflib.URIRef(ns.xsd.boolean), rdflib.RDFS.subClassOf, rdflib.URIRef(ns.bsfs.Literal)),
+            (rdflib.URIRef(ns.bse.shared), rdflib.RDFS.subClassOf, rdflib.URIRef(ns.bsfs.Predicate)),
+            (rdflib.URIRef(ns.bse.partOf), rdflib.RDFS.subClassOf, rdflib.URIRef(ns.bsfs.Predicate)),
+            (rdflib.URIRef('https://schema.bsfs.io/core/Tag#usedIn'), rdflib.RDFS.subClassOf, rdflib.URIRef(ns.bsfs.Predicate)),
+            (rdflib.URIRef('https://schema.bsfs.io/core/Collection#tag'), rdflib.RDFS.subClassOf, rdflib.URIRef(ns.bsfs.Predicate)),
+            (rdflib.URIRef('https://schema.bsfs.io/core/Tag#principal'), rdflib.RDFS.subClassOf, rdflib.URIRef(ns.bsfs.Predicate)),
+            })
         # add some instances of the new classes
         p_partOf = curr.predicate(ns.bse.partOf)
         p_shared = curr.predicate(ns.bse.shared)
-        p_usedIn = curr.predicate('http://bsfs.ai/schema/Tag#usedIn')
-        p_ctag = curr.predicate('http://bsfs.ai/schema/Collection#tag')
-        p_principal = curr.predicate('http://bsfs.ai/schema/Tag#principal')
+        p_usedIn = curr.predicate('https://schema.bsfs.io/core/Tag#usedIn')
+        p_ctag = curr.predicate('https://schema.bsfs.io/core/Collection#tag')
+        p_principal = curr.predicate('https://schema.bsfs.io/core/Tag#principal')
         store.create(curr.node(ns.bsfs.Collection), {URI('http://example.com/me/collection#1234'), URI('http://example.com/me/collection#4321')})
         # add some more triples
         store.set(curr.node(ns.bsfs.Entity), ent_ids, p_shared, {True})
@@ -248,6 +281,14 @@ class TestSparqlStore(unittest.TestCase):
             {URI('http://example.com/me/collection#1234')})
         # new instances are now in the graph
         self.assertSetEqual(set(store._graph), instances | {
+            # same old schema hierarchy
+            (rdflib.URIRef(ns.bsfs.Collection), rdflib.RDFS.subClassOf, rdflib.URIRef(ns.bsfs.Node)),
+            (rdflib.URIRef(ns.xsd.boolean), rdflib.RDFS.subClassOf, rdflib.URIRef(ns.bsfs.Literal)),
+            (rdflib.URIRef(ns.bse.shared), rdflib.RDFS.subClassOf, rdflib.URIRef(ns.bsfs.Predicate)),
+            (rdflib.URIRef(ns.bse.partOf), rdflib.RDFS.subClassOf, rdflib.URIRef(ns.bsfs.Predicate)),
+            (rdflib.URIRef('https://schema.bsfs.io/core/Tag#usedIn'), rdflib.RDFS.subClassOf, rdflib.URIRef(ns.bsfs.Predicate)),
+            (rdflib.URIRef('https://schema.bsfs.io/core/Collection#tag'), rdflib.RDFS.subClassOf, rdflib.URIRef(ns.bsfs.Predicate)),
+            (rdflib.URIRef('https://schema.bsfs.io/core/Tag#principal'), rdflib.RDFS.subClassOf, rdflib.URIRef(ns.bsfs.Predicate)),
             # collections
             (rdflib.URIRef('http://example.com/me/collection#1234'), rdflib.RDF.type, rdflib.URIRef(ns.bsfs.Collection)),
             (rdflib.URIRef('http://example.com/me/collection#4321'), rdflib.RDF.type, rdflib.URIRef(ns.bsfs.Collection)),
@@ -268,19 +309,21 @@ class TestSparqlStore(unittest.TestCase):
 
 
         # remove some classes from the schema
-        curr = _schema.Schema.from_string('''
+        curr = bsc.from_string('''
             prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
             prefix xsd: <http://www.w3.org/2001/XMLSchema#>
-            prefix bsfs: <http://bsfs.ai/schema/>
-            prefix bse: <http://bsfs.ai/schema/Entity#>
-            prefix bst: <http://bsfs.ai/schema/Tag#>
+            prefix bsfs: <https://schema.bsfs.io/core/>
+            prefix bse: <https://schema.bsfs.io/core/Entity#>
+            prefix bst: <https://schema.bsfs.io/core/Tag#>
+            prefix bsl: <https://schema.bsfs.io/core/Literal/>
 
             bsfs:Entity rdfs:subClassOf bsfs:Node .
             bsfs:Tag rdfs:subClassOf bsfs:Node .
             bsfs:User rdfs:subClassOf bsfs:Node .
 
             xsd:boolean rdfs:subClassOf bsfs:Literal .
-            xsd:integer rdfs:subClassOf bsfs:Literal .
+            bsl:Number rdfs:subClassOf bsfs:Literal .
+            xsd:integer rdfs:subClassOf bsl:Number .
 
             bse:filesize rdfs:subClassOf bsfs:Predicate ;
                 rdfs:domain bsfs:Entity ;
@@ -316,6 +359,21 @@ class TestSparqlStore(unittest.TestCase):
         self.assertEqual(store.schema, curr)
         # instances of old classes were removed
         self.assertSetEqual(set(store._graph), {
+            # schema hierarchy
+            (rdflib.URIRef(ns.bsfs.Entity), rdflib.RDFS.subClassOf, rdflib.URIRef(ns.bsfs.Node)),
+            (rdflib.URIRef(ns.bsfs.Tag), rdflib.RDFS.subClassOf, rdflib.URIRef(ns.bsfs.Node)),
+            (rdflib.URIRef(ns.bsfs.User), rdflib.RDFS.subClassOf, rdflib.URIRef(ns.bsfs.Node)),
+            (rdflib.URIRef(ns.xsd.boolean), rdflib.RDFS.subClassOf, rdflib.URIRef(ns.bsfs.Literal)),
+            (rdflib.URIRef(ns.bsl.Array), rdflib.RDFS.subClassOf, rdflib.URIRef(ns.bsfs.Literal)),
+            (rdflib.URIRef(ns.bsl.BinaryBlob), rdflib.RDFS.subClassOf, rdflib.URIRef(ns.bsfs.Literal)),
+            (rdflib.URIRef(ns.bsl.Array.Feature), rdflib.RDFS.subClassOf, rdflib.URIRef(ns.bsl.Array)),
+            (rdflib.URIRef(ns.bsl.Number), rdflib.RDFS.subClassOf, rdflib.URIRef(ns.bsfs.Literal)),
+            (rdflib.URIRef(ns.bsl.Time), rdflib.RDFS.subClassOf, rdflib.URIRef(ns.bsfs.Literal)),
+            (rdflib.URIRef(ns.xsd.integer), rdflib.RDFS.subClassOf, rdflib.URIRef(ns.bsl.Number)),
+            (rdflib.URIRef(ns.bse.shared), rdflib.RDFS.subClassOf, rdflib.URIRef(ns.bsfs.Predicate)),
+            (rdflib.URIRef(ns.bse.tag), rdflib.RDFS.subClassOf, rdflib.URIRef(ns.bsfs.Predicate)),
+            (rdflib.URIRef(ns.bse.filesize), rdflib.RDFS.subClassOf, rdflib.URIRef(ns.bsfs.Predicate)),
+            (rdflib.URIRef('https://schema.bsfs.io/core/Tag#principal'), rdflib.RDFS.subClassOf, rdflib.URIRef(ns.bsfs.Predicate)),
             # node instances
             (rdflib.URIRef('http://example.com/me/entity#1234'), rdflib.RDF.type, rdflib.URIRef(ns.bsfs.Entity)),
             (rdflib.URIRef('http://example.com/me/entity#4321'), rdflib.RDF.type, rdflib.URIRef(ns.bsfs.Entity)),
@@ -342,12 +400,31 @@ class TestSparqlStore(unittest.TestCase):
         class Foo(): pass
         self.assertRaises(TypeError, setattr, store, 'schema', Foo())
 
+        # cannot define features w/o known distance function
+        invalid = bsc.from_string('''
+            prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
+            prefix xsd: <http://www.w3.org/2001/XMLSchema#>
+            prefix bsfs: <https://schema.bsfs.io/core/>
+            prefix bse: <https://schema.bsfs.io/core/Entity#>
+            prefix bsl: <https://schema.bsfs.io/core/Literal/>
+            prefix bsa: <https://schema.bsfs.io/core/Literal/Array/>
+
+            bsl:Array rdfs:subClassOf bsfs:Literal .
+            bsa:Feature rdfs:subClassOf bsl:Array .
+
+            bsfs:Colors rdfs:subClassOf bsa:Feature ;
+                bsfs:dimension "4"^^xsd:integer ;
+                bsfs:distance bsfs:foobar .
+
+            ''')
+        self.assertRaises(errors.UnsupportedError, setattr, store, 'schema', invalid)
+
         # cannot migrate to incompatible schema
-        invalid = _schema.Schema.from_string('''
+        invalid = bsc.from_string('''
             prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
             prefix xsd: <http://www.w3.org/2001/XMLSchema#>
-            prefix bsfs: <http://bsfs.ai/schema/>
-            prefix bse: <http://bsfs.ai/schema/Entity#>
+            prefix bsfs: <https://schema.bsfs.io/core/>
+            prefix bse: <https://schema.bsfs.io/core/Entity#>
 
             bsfs:Entity rdfs:subClassOf bsfs:Node .
             bsfs:Tag rdfs:subClassOf bsfs:Entity . # inconsistent with previous tag definition
@@ -359,11 +436,11 @@ class TestSparqlStore(unittest.TestCase):
 
             ''')
         self.assertRaises(errors.ConsistencyError, setattr, store, 'schema', invalid)
-        invalid = _schema.Schema.from_string('''
+        invalid = bsc.from_string('''
             prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>
             prefix xsd: <http://www.w3.org/2001/XMLSchema#>
-            prefix bsfs: <http://bsfs.ai/schema/>
-            prefix bse: <http://bsfs.ai/schema/Entity#>
+            prefix bsfs: <https://schema.bsfs.io/core/>
+            prefix bse: <https://schema.bsfs.io/core/Entity#>
 
             bsfs:Entity rdfs:subClassOf bsfs:Node .
             bsfs:User rdfs:subClassOf bsfs:Node .
@@ -390,7 +467,7 @@ class TestSparqlStore(unittest.TestCase):
         ent_ids = {URI('http://example.com/me/entity#1234'), URI('http://example.com/me/entity#4321')}
         tag_ids = {URI('http://example.com/me/tag#1234'), URI('http://example.com/me/tag#4321')}
         # target instances
-        instances = {
+        instances = self.schema_triples | {
             # node instances
             (rdflib.URIRef('http://example.com/me/entity#1234'), rdflib.RDF.type, rdflib.URIRef(ns.bsfs.Entity)),
             (rdflib.URIRef('http://example.com/me/entity#4321'), rdflib.RDF.type, rdflib.URIRef(ns.bsfs.Entity)),
@@ -416,7 +493,7 @@ class TestSparqlStore(unittest.TestCase):
 
         # rollback undoes previous changes
         store.rollback()
-        self.assertSetEqual(set(store._graph), set())
+        self.assertSetEqual(set(store._graph), self.schema_triples)
 
         # add some data once more
         store.create(ent_type, ent_ids)
@@ -455,6 +532,109 @@ class TestSparqlStore(unittest.TestCase):
             (rdflib.URIRef('http://example.com/me/entity#4321'), rdflib.URIRef(p_filesize.uri), rdflib.Literal(1234, datatype=rdflib.XSD.integer)),
             })
 
+    def test_get(self):
+        # store setup
+        store = SparqlStore.Open()
+        store.schema = self.schema
+        ent_type = self.schema.node(ns.bsfs.Entity)
+        tag_type = self.schema.node(ns.bsfs.Tag)
+        ent_ids = {URI('http://example.com/me/entity#1234'), URI('http://example.com/me/entity#4321')}
+        tag_ids = {URI('http://example.com/me/tag#1234'), URI('http://example.com/me/tag#4321')}
+        store.create(ent_type, ent_ids)
+        store.create(tag_type, tag_ids)
+        store.set(ent_type, ent_ids, self.schema.predicate(ns.bse.tag), tag_ids)
+        store.set(ent_type, {URI('http://example.com/me/entity#1234')}, self.schema.predicate(ns.bse.filesize), {1234})
+        store.set(ent_type, {URI('http://example.com/me/entity#4321')}, self.schema.predicate(ns.bse.filesize), {4321})
+        # node_type must be in the schema
+        self.assertRaises(errors.ConsistencyError, set, store.get(self.schema.node(ns.bsfs.Node).child(ns.bsfs.Invalid), ast.filter.IsIn(ent_ids)))
+        # query must be a filter expression
+        class Foo(): pass
+        self.assertRaises(TypeError, set, store.get(ent_type, 1234))
+        self.assertRaises(TypeError, set, store.get(ent_type, '1234'))
+        self.assertRaises(TypeError, set, store.get(ent_type, Foo()))
+        # run some queries
+        self.assertSetEqual(set(store.get(tag_type, ast.filter.IsIn(tag_ids))), tag_ids)
+        self.assertSetEqual(set(store.get(ent_type, ast.filter.Any(ns.bse.tag, ast.filter.IsIn(tag_ids)))), ent_ids)
+        self.assertSetEqual(set(store.get(ent_type, ast.filter.IsIn(tag_ids))), set())
+        # invalid queries raise error
+        self.assertRaises(errors.ConsistencyError, set, store.get(tag_type, ast.filter.Any(ns.bse.filesize, ast.filter.Equals(1234))))
+        self.assertRaises(errors.BackendError, set, store.get(ent_type, ast.filter.Equals('http://example.com/me/entity#1234')))
+        # run some more complex query
+        q = store.get(tag_type, ast.filter.Any(ast.filter.Predicate(ns.bse.tag, reverse=True),
+            ast.filter.Any(ns.bse.filesize,
+                ast.filter.LessThan(2000))))
+        self.assertSetEqual(set(q), tag_ids)
+
+
+    def test_fetch(self):
+        # store setup
+        store = SparqlStore.Open()
+        store.schema = self.schema
+        # add instances
+        ent_type = self.schema.node(ns.bsfs.Entity)
+        tag_type = self.schema.node(ns.bsfs.Tag)
+        ent_ids = {URI('http://example.com/me/entity#1234'), URI('http://example.com/me/entity#4321')}
+        tag_ids = {URI('http://example.com/me/tag#1234'), URI('http://example.com/me/tag#4321')}
+        store.create(ent_type, ent_ids)
+        store.create(tag_type, tag_ids)
+        store.set(ent_type, ent_ids, self.schema.predicate(ns.bse.tag), tag_ids)
+        store.set(ent_type, {URI('http://example.com/me/entity#1234')}, self.schema.predicate(ns.bse.filesize), {1234})
+        store.set(ent_type, {URI('http://example.com/me/entity#4321')}, self.schema.predicate(ns.bse.filesize), {4321})
+        store.set(ent_type, {URI('http://example.com/me/entity#1234')}, self.schema.predicate(ns.bse.comment), {'hello world'})
+        # node_type must be a node from the schema
+        self.assertRaises(errors.ConsistencyError, list, store.fetch(self.schema.literal(ns.bsfs.Literal),
+            ast.filter.FilterExpression(), ast.fetch.FetchExpression()))
+        self.assertRaises(errors.ConsistencyError, list, store.fetch(self.schema.node(ns.bsfs.Node).child(ns.bsfs.Invalid),
+            ast.filter.FilterExpression(), ast.fetch.FetchExpression()))
+        # requires a filter and a fetch query
+        self.assertRaises(TypeError, list, store.fetch(self.schema.node(ns.bsfs.Entity), None, ast.fetch.FetchExpression()))
+        self.assertRaises(TypeError, list, store.fetch(self.schema.node(ns.bsfs.Entity), 1234, ast.fetch.FetchExpression()))
+        self.assertRaises(TypeError, list, store.fetch(self.schema.node(ns.bsfs.Entity), 'hello', ast.fetch.FetchExpression()))
+        self.assertRaises(TypeError, list, store.fetch(self.schema.node(ns.bsfs.Entity), ast.filter.FilterExpression(), None))
+        self.assertRaises(TypeError, list, store.fetch(self.schema.node(ns.bsfs.Entity), ast.filter.FilterExpression(), 1234))
+        self.assertRaises(TypeError, list, store.fetch(self.schema.node(ns.bsfs.Entity), ast.filter.FilterExpression(), 'hello'))
+        # fetch emits triples
+        self.assertSetEqual(set(store.fetch(self.schema.node(ns.bsfs.Entity),
+            ast.filter.Is('http://example.com/me/entity#1234'),
+            ast.fetch.Value(ns.bse.filesize, 'filesize'),
+            )), {
+            (URI('http://example.com/me/entity#1234'), 'filesize', 1234),
+            })
+        # fetch respects filter query
+        self.assertSetEqual(set(store.fetch(self.schema.node(ns.bsfs.Entity),
+            ast.filter.IsIn('http://example.com/me/entity#1234', 'http://example.com/me/entity#4321'),
+            ast.fetch.Value(ns.bse.filesize, 'filesize'),
+            )), {
+            (URI('http://example.com/me/entity#1234'), 'filesize', 1234),
+            (URI('http://example.com/me/entity#4321'), 'filesize', 4321),
+            })
+        # fetch ignores missing data
+        self.assertSetEqual(set(store.fetch(self.schema.node(ns.bsfs.Entity),
+            ast.filter.IsIn('http://example.com/me/entity#1234', 'http://example.com/me/entity#4321'),
+            ast.fetch.Value(ns.bse.comment, 'comment'),
+            )), {
+            (URI('http://example.com/me/entity#1234'), 'comment', 'hello world'),
+            })
+        # fetch emits all triples
+        self.assertSetEqual(set(store.fetch(self.schema.node(ns.bsfs.Entity),
+            ast.filter.Is('http://example.com/me/entity#1234'),
+            ast.fetch.All(
+                ast.fetch.Value(ns.bse.filesize, 'filesize'),
+                ast.fetch.Node(ns.bse.tag, 'tag'),
+                )
+            )), {
+            (URI('http://example.com/me/entity#1234'), 'filesize', 1234),
+            (URI('http://example.com/me/entity#1234'), 'tag', URI('http://example.com/me/tag#1234')),
+            (URI('http://example.com/me/entity#1234'), 'tag', URI('http://example.com/me/tag#4321')),
+            })
+        # triples do not repeat
+        triples = list(store.fetch(self.schema.node(ns.bsfs.Entity), ast.filter.Is('http://example.com/me/entity#1234'),
+            ast.fetch.All(
+                ast.fetch.Value(ns.bse.filesize, 'filesize'),
+                ast.fetch.Node(ns.bse.tag, 'tag'),
+                )
+            ))
+        self.assertEqual(len(triples), 3)
 
     def test_exists(self):
         # store setup
@@ -501,20 +681,24 @@ class TestSparqlStore(unittest.TestCase):
         store.schema = self.schema
 
         # node type must be valid
-        self.assertRaises(errors.ConsistencyError, store.create, self.schema.node(ns.bsfs.Entity).get_child(ns.bsfs.invalid), {
+        self.assertRaises(errors.ConsistencyError, store.create, self.schema.node(ns.bsfs.Entity).child(ns.bsfs.invalid), {
             URI('http://example.com/me/entity#1234'), URI('http://example.com/me/entity#4321')})
 
+        # guid must be valid
+        self.assertRaises(ValueError, store.create, self.schema.node(ns.bsfs.Entity), {'http://example.com/me/foo and bar'})
+
         # can create some nodes
         ent_type = store.schema.node(ns.bsfs.Entity)
         store.create(ent_type, {URI('http://example.com/me/entity#1234'), URI('http://example.com/me/entity#4321')})
-        self.assertSetEqual(set(store._graph), {
+        self.assertSetEqual(set(store._graph), self.schema_triples | {
+            # instances
             (rdflib.URIRef('http://example.com/me/entity#1234'), rdflib.RDF.type, rdflib.URIRef(ns.bsfs.Entity)),
             (rdflib.URIRef('http://example.com/me/entity#4321'), rdflib.RDF.type, rdflib.URIRef(ns.bsfs.Entity)),
             })
 
         # existing nodes are skipped
         store.create(ent_type, {URI('http://example.com/me/entity#1234'), URI('http://example.com/me/entity#5678')})
-        self.assertSetEqual(set(store._graph), {
+        self.assertSetEqual(set(store._graph), self.schema_triples | {
             # previous triples
             (rdflib.URIRef('http://example.com/me/entity#1234'), rdflib.RDF.type, rdflib.URIRef(ns.bsfs.Entity)),
             (rdflib.URIRef('http://example.com/me/entity#4321'), rdflib.RDF.type, rdflib.URIRef(ns.bsfs.Entity)),
@@ -525,7 +709,7 @@ class TestSparqlStore(unittest.TestCase):
         # can create nodes of a different type
         tag_type = store.schema.node(ns.bsfs.Tag)
         store.create(tag_type, {URI('http://example.com/me/tag#1234'), URI('http://example.com/me/tag#4321')})
-        self.assertSetEqual(set(store._graph), {
+        self.assertSetEqual(set(store._graph), self.schema_triples | {
             # previous triples
             (rdflib.URIRef('http://example.com/me/entity#1234'), rdflib.RDF.type, rdflib.URIRef(ns.bsfs.Entity)),
             (rdflib.URIRef('http://example.com/me/entity#4321'), rdflib.RDF.type, rdflib.URIRef(ns.bsfs.Entity)),
@@ -538,7 +722,7 @@ class TestSparqlStore(unittest.TestCase):
         # creation does not change types of existing nodes
         tag_type = store.schema.node(ns.bsfs.Tag)
         store.create(tag_type, {URI('http://example.com/me/entity#1234'), URI('http://example.com/me/entity#4321')})
-        self.assertSetEqual(set(store._graph), {
+        self.assertSetEqual(set(store._graph), self.schema_triples | {
             # previous triples
             (rdflib.URIRef('http://example.com/me/entity#1234'), rdflib.RDF.type, rdflib.URIRef(ns.bsfs.Entity)),
             (rdflib.URIRef('http://example.com/me/entity#4321'), rdflib.RDF.type, rdflib.URIRef(ns.bsfs.Entity)),
@@ -562,7 +746,7 @@ class TestSparqlStore(unittest.TestCase):
         p_comment = store.schema.predicate(ns.bse.comment)
         p_author = store.schema.predicate(ns.bse.author)
         p_tag = store.schema.predicate(ns.bse.tag)
-        p_invalid = store.schema.predicate(ns.bsfs.Predicate).get_child(ns.bsfs.foo, range=store.schema.node(ns.bsfs.Tag))
+        p_invalid = store.schema.predicate(ns.bsfs.Predicate).child(ns.bsfs.foo, range=store.schema.node(ns.bsfs.Tag))
         # create node instances
         ent_ids = {
             URI('http://example.com/me/entity#1234'),
@@ -585,12 +769,15 @@ class TestSparqlStore(unittest.TestCase):
         store.create(user_type, user_ids)
 
         # invalid node_type is not permitted
-        self.assertRaises(errors.ConsistencyError, store.set, self.schema.node(ns.bsfs.Node).get_child(ns.bse.foo),
+        self.assertRaises(errors.ConsistencyError, store.set, self.schema.node(ns.bsfs.Node).child(ns.bse.foo),
             ent_ids, p_comment, {'hello world'})
 
         # invalid predicate is not permitted
         self.assertRaises(errors.ConsistencyError, store.set, ent_type, ent_ids, p_invalid, {'http://example.com/me/tag#1234'})
 
+        # invalid guid is not permitted
+        self.assertRaises(ValueError, store.set, ent_type, {'http://example.com/me/foo and bar'}, p_filesize, {1234})
+
         # predicate must match node_type
         self.assertRaises(errors.ConsistencyError, store.set, tag_type, tag_ids, p_filesize, {1234})
 
@@ -760,6 +947,23 @@ class TestSparqlStore(unittest.TestCase):
         # inexistent guids
         self.assertRaises(errors.InstanceError, store.set, ent_type, {URI('http://example.com/me/entity#foobar')}, p_comment, {'xyz'})
 
+        # BinaryBlob values are base64 encoded
+        p_asset = store.schema.predicate(ns.bse.asset)
+        store.set(ent_type, ent_ids, p_asset, {bytes(range(128)), bytes(range(128, 256))})
+        blob1 = rdflib.Literal('AAECAwQFBgcICQoLDA0ODxAREhMUFRYXGBkaGxwdHh8gISIjJCUmJygpKissLS4vMDEyMzQ1Njc4OTo7PD0+P0BBQkNERUZHSElKS0xNTk9QUVJTVFVWV1hZWltcXV5fYGFiY2RlZmdoaWprbG1ub3BxcnN0dXZ3eHl6e3x9fn8=',
+            datatype=rdflib.URIRef(ns.bsl.BinaryBlob))
+        blob2 = rdflib.Literal('gIGCg4SFhoeIiYqLjI2Oj5CRkpOUlZaXmJmam5ydnp+goaKjpKWmp6ipqqusra6vsLGys7S1tre4ubq7vL2+v8DBwsPExcbHyMnKy8zNzs/Q0dLT1NXW19jZ2tvc3d7f4OHi4+Tl5ufo6err7O3u7/Dx8vP09fb3+Pn6+/z9/v8=',
+            datatype=rdflib.URIRef(ns.bsl.BinaryBlob))
+        self.assertTrue(set(store._graph).issuperset({
+            (rdflib.URIRef('http://example.com/me/entity#1234'), rdflib.URIRef(p_asset.uri), blob1),
+            (rdflib.URIRef('http://example.com/me/entity#1234'), rdflib.URIRef(p_asset.uri), blob2),
+            (rdflib.URIRef('http://example.com/me/entity#4321'), rdflib.URIRef(p_asset.uri), blob1),
+            (rdflib.URIRef('http://example.com/me/entity#4321'), rdflib.URIRef(p_asset.uri), blob2),
+            }))
+        # lit.value returns the original bytes value
+        self.assertSetEqual({lit.value for lit in store._graph.objects(None, rdflib.URIRef(p_asset.uri))},
+            {bytes(range(128)), bytes(range(128, 256))})
+
 
 ## main ##
 
diff --git a/test/triple_store/sparql/test_utils.py b/test/triple_store/sparql/test_utils.py
new file mode 100644
index 0000000..44a1299
--- /dev/null
+++ b/test/triple_store/sparql/test_utils.py
@@ -0,0 +1,152 @@
+
+# standard imports
+import operator
+import re
+import unittest
+
+# external imports
+import rdflib
+
+# bsie imports
+from bsfs.namespace import ns
+
+# objects to test
+from bsfs.triple_store.sparql.utils import GenHopName, Query
+
+
+## code ##
+
+ns.bse = ns.bsfs.Entity()
+
+class TestGenHopName(unittest.TestCase):
+    def test_next(self):
+        # baseline
+        self.assertEqual(next(GenHopName(prefix='?foo', start=123)), '?foo123')
+        # respects prefix
+        self.assertEqual(next(GenHopName(prefix='?bar', start=123)), '?bar123')
+        # respects start
+        self.assertEqual(next(GenHopName(prefix='?foo', start=321)), '?foo321')
+        # counts up
+        cnt = GenHopName(prefix='?foo', start=998)
+        self.assertEqual(next(cnt), '?foo998')
+        self.assertEqual(next(cnt), '?foo999')
+        self.assertEqual(next(cnt), '?foo1000')
+        self.assertEqual(next(cnt), '?foo1001')
+
+    def test_essentials(self):
+        # can get the prefix
+        self.assertEqual(GenHopName(prefix='?foo', start=123).prefix, '?foo')
+        # can get the counter
+        self.assertEqual(GenHopName(prefix='?foo', start=123).curr, 122)
+
+
+class TestQuery(unittest.TestCase):
+    def setUp(self):
+        self.root_type = 'https://schema.bsfs.io/core/Entity'
+        self.root_head = '?root'
+        self.select = (('?head', 'name'), )
+        self.where = f'?root <{ns.bse.tag}> ?head'
+
+    def test_essentials(self):
+        # can access members
+        q = Query(self.root_type, self.root_head, self.select, self.where)
+        self.assertEqual(q.root_type, self.root_type)
+        self.assertEqual(q.root_head, self.root_head)
+        self.assertEqual(q.select, self.select)
+        self.assertEqual(q.where, self.where)
+        # comparison
+        self.assertEqual(q, Query(self.root_type, self.root_head, self.select, self.where))
+        self.assertEqual(hash(q), hash(Query(self.root_type, self.root_head, self.select, self.where)))
+        # comparison respects root_type
+        self.assertNotEqual(q, Query('https://schema.bsfs.io/core/Tag', self.root_head, self.select, self.where))
+        self.assertNotEqual(hash(q), hash(Query('https://schema.bsfs.io/core/Tag', self.root_head, self.select, self.where)))
+        # comparison respects root_head
+        self.assertNotEqual(q, Query(self.root_type, '?foo', self.select, self.where))
+        self.assertNotEqual(hash(q), hash(Query(self.root_type, '?foo', self.select, self.where)))
+        # comparison respects select
+        self.assertNotEqual(q, Query(self.root_type, self.root_head, (('?head', 'foo'), ), self.where))
+        self.assertNotEqual(hash(q), hash(Query(self.root_type, self.root_head, (('?head', 'foo'), ), self.where)))
+        # comparison respects where
+        self.assertNotEqual(q, Query(self.root_type, self.root_head, self.select, '?root bse:filename ?head'))
+        self.assertNotEqual(hash(q), hash(Query(self.root_type, self.root_head, self.select, '?root bse:filename ?head')))
+        # string conversion
+        self.assertEqual(str(q), q.query)
+        self.assertEqual(repr(q), "Query(https://schema.bsfs.io/core/Entity, ?root, (('?head', 'name'),), ?root <https://schema.bsfs.io/core/Entity#tag> ?head)")
+
+    def test_add(self):
+        q = Query(self.root_type, self.root_head, self.select, self.where)
+        # can only add a query
+        self.assertRaises(TypeError, operator.add, q, 1234)
+        self.assertRaises(TypeError, operator.add, q, 'foobar')
+        # root type and head must match
+        self.assertRaises(ValueError, operator.add, q, Query('https://schema.bsfs.io/core/Node/Tag', self.root_head))
+        self.assertRaises(ValueError, operator.add, q, Query(self.root_type, '?foobar'))
+        # select and were are combined
+        combo = q + Query(self.root_type, self.root_head, (('?foo', 'bar'), ), f'?root <{ns.bse.filename}> ?foo')
+        self.assertEqual(combo.select, (('?head', 'name'), ('?foo', 'bar')))
+        self.assertEqual(combo.where, f'?root <{ns.bse.tag}> ?head . ?root <{ns.bse.filename}> ?foo')
+        # select can be empty
+        combo = q + Query(self.root_type, self.root_head, None, f'?root <{ns.bse.filename}> ?foo')
+        self.assertEqual(combo.select, (('?head', 'name'), ))
+        combo = Query(self.root_type, self.root_head, None, f'?root <{ns.bse.filename}> ?foo') + q
+        self.assertEqual(combo.select, (('?head', 'name'), ))
+        combo = Query(self.root_type, self.root_head, None, self.where) + Query(self.root_type, self.root_head, None, f'?root <{ns.bse.filename}> ?foo')
+        self.assertEqual(combo.select, tuple())
+        # where can be empty
+        combo = q + Query(self.root_type, self.root_head, (('?foo', 'bar'), ))
+        self.assertEqual(combo.where, self.where)
+        combo = Query(self.root_type, self.root_head, (('?foo', 'bar'), )) + q
+        self.assertEqual(combo.where, self.where)
+        combo = Query(self.root_type, self.root_head, self.select) + Query(self.root_type, self.root_head, (('?foo', 'bar'), ))
+        self.assertEqual(combo.where, '')
+
+    def test_names(self):
+        self.assertEqual(Query(self.root_type, self.root_head, (('?head', 'name'), ), self.where).names,
+            ('name', ))
+        self.assertEqual(Query(self.root_type, self.root_head, (('?head', 'name'), ('?foo', 'bar')), self.where).names,
+            ('name', 'bar'))
+
+    def test_query(self):
+        def normalize(value):
+            value = value.strip()
+            value = value.lower()
+            value = value.replace(r'\n', ' ')
+            value, _ = re.subn('\s\s+', ' ', value)
+            return value
+        # query composes a valid query
+        q = Query(self.root_type, self.root_head, self.select, self.where)
+        self.assertEqual(normalize(q.query), normalize(f'select distinct ?root (?head as ?name) where {{ ?root <{ns.rdf.type}>/<{ns.rdfs.subClassOf}>* <https://schema.bsfs.io/core/Entity> . ?root <{ns.bse.tag}> ?head }} order by str(?root)'))
+        # select and where are optional
+        q = Query(self.root_type, self.root_head)
+        self.assertEqual(normalize(q.query), normalize(f'select distinct ?root where {{ ?root <{ns.rdf.type}>/<{ns.rdfs.subClassOf}>* <https://schema.bsfs.io/core/Entity> . }} order by str(?root)'))
+        # select and where need not to correspond
+        q = Query(self.root_type, self.root_head, (('?head', 'name'), ))
+        self.assertEqual(normalize(q.query), normalize(f'select distinct ?root (?head as ?name) where {{ ?root <{ns.rdf.type}>/<{ns.rdfs.subClassOf}>* <https://schema.bsfs.io/core/Entity> . }} order by str(?root)'))
+        # query is used for string representation
+        self.assertEqual(str(q), q.query)
+
+    def test_call(self):
+        graph = rdflib.Graph()
+        # schema
+        graph.add((rdflib.URIRef('https://schema.bsfs.io/core/Document'), rdflib.URIRef(ns.rdfs.subClassOf), rdflib.URIRef('https://schema.bsfs.io/core/Entity')))
+        # nodes
+        graph.add((rdflib.URIRef('http://example.com/entity#1234'), rdflib.URIRef(ns.rdf.type), rdflib.URIRef('https://schema.bsfs.io/core/Entity')))
+        graph.add((rdflib.URIRef('http://example.com/doc#1234'), rdflib.URIRef(ns.rdf.type), rdflib.URIRef('https://schema.bsfs.io/core/Document')))
+        # links
+        graph.add((rdflib.URIRef('http://example.com/entity#1234'), rdflib.URIRef(ns.bse.tag), rdflib.Literal('tag#1234', datatype=rdflib.XSD.string)))
+        graph.add((rdflib.URIRef('http://example.com/doc#1234'), rdflib.URIRef(ns.bse.tag), rdflib.Literal('tag#1234', datatype=rdflib.XSD.string)))
+        # run query on a given graph
+        query = Query(self.root_type, self.root_head, self.select, self.where)
+        self.assertSetEqual(set(query(graph)), {
+            (rdflib.URIRef('http://example.com/entity#1234'), rdflib.Literal('tag#1234', datatype=rdflib.XSD.string)),
+            (rdflib.URIRef('http://example.com/doc#1234'), rdflib.Literal('tag#1234', datatype=rdflib.XSD.string)),
+            })
+        # query actually considers the passed graph
+        self.assertSetEqual(set(query(rdflib.Graph())), set())
+
+## main ##
+
+if __name__ == '__main__':
+    unittest.main()
+
+## EOF ##
diff --git a/test/triple_store/test_base.py b/test/triple_store/test_base.py
index a4b0559..4c4a9b6 100644
--- a/test/triple_store/test_base.py
+++ b/test/triple_store/test_base.py
@@ -1,9 +1,4 @@
-"""
 
-Part of the bsfs test suite.
-A copy of the license is provided with the project.
-Author: Matthias Baumgartner, 2022
-"""
 # imports
 import unittest
 
@@ -35,6 +30,12 @@ class DummyBase(TripleStoreBase):
     def schema(self, schema):
         pass
 
+    def get(self, node_type, query):
+        pass
+
+    def fetch(self, node_type, filter, fetch):
+        pass
+
     def exists(self, node_type, guids):
         pass
 
diff --git a/test/utils/test_commons.py b/test/utils/test_commons.py
index ce73788..29e3046 100644
--- a/test/utils/test_commons.py
+++ b/test/utils/test_commons.py
@@ -1,14 +1,9 @@
-"""
 
-Part of the tagit test suite.
-A copy of the license is provided with the project.
-Author: Matthias Baumgartner, 2022
-"""
 # imports
 import unittest
 
 # objects to test
-from bsfs.utils.commons import typename
+from bsfs.utils.commons import typename, normalize_args
 
 
 ## code ##
@@ -21,6 +16,21 @@ class TestCommons(unittest.TestCase):
         self.assertEqual(typename(123), 'int')
         self.assertEqual(typename(None), 'NoneType')
 
+    def test_normalize_args(self):
+        # one argument
+        self.assertEqual(normalize_args(1), (1, ))
+        # pass as arguments
+        self.assertEqual(normalize_args(1,2,3), (1,2,3))
+        # pass as iterator
+        self.assertEqual(normalize_args(iter([1,2,3])), (1,2,3))
+        # pass as generator
+        self.assertEqual(normalize_args((i for i in range(1, 4))), (1,2,3))
+        self.assertEqual(normalize_args(i for i in range(1, 4)), (1,2,3)) # w/o brackets
+        # pass as iterable
+        self.assertEqual(normalize_args([1,2,3]), (1,2,3))
+        # pass an iterable with a single item
+        self.assertEqual(normalize_args([1]), (1, ))
+
 
 
 ## main ##
diff --git a/test/utils/test_uri.py b/test/utils/test_uri.py
index 770e65a..1c4c9f9 100644
--- a/test/utils/test_uri.py
+++ b/test/utils/test_uri.py
@@ -1,9 +1,4 @@
-"""
 
-Part of the tagit test suite.
-A copy of the license is provided with the project.
-Author: Matthias Baumgartner, 2022
-"""
 # imports
 import operator
 import unittest
@@ -40,6 +35,16 @@ class TestURI(unittest.TestCase):
         self.assertTrue(URI.is_parseable('telnet://192.0.2.16:80/'))
         self.assertTrue(URI.is_parseable('urn:oasis:names:specification:docbook:dtd:xml:4.1.2'))
 
+        # some characters are prohibited
+        self.assertFalse(URI.is_parseable('http://example.com/foo<bar'))
+        self.assertFalse(URI.is_parseable('http://example.com/foo>bar'))
+        self.assertFalse(URI.is_parseable('http://example.com/foo bar'))
+        self.assertFalse(URI.is_parseable('http://example.com/foo{bar'))
+        self.assertFalse(URI.is_parseable('http://example.com/foo}bar'))
+        self.assertFalse(URI.is_parseable('http://example.com/foo|bar'))
+        self.assertFalse(URI.is_parseable('http://example.com/foo^bar'))
+        self.assertFalse(URI.is_parseable('http://example.com/foo\\bar'))
+
         # uri cannot end with a scheme delimiter
         self.assertFalse(URI.is_parseable('http://'))
         # port must be a number
@@ -164,10 +169,10 @@ class TestURI(unittest.TestCase):
 
     def test_overloaded(self):
         # composition
-        self.assertIsInstance(URI('http://user@www.example.com:1234/{}/path1?{}#fragment') + 'hello', URI)
-        self.assertIsInstance(URI('http://user@www.example.com:1234/{}/path1?{}#fragment') * 2, URI)
-        self.assertIsInstance(2 * URI('http://user@www.example.com:1234/{}/path1?{}#fragment'), URI) # rmul
-        self.assertIsInstance(URI('http://user@www.example.com:1234/{}/path1?{}#fragment').join(['hello', 'world']) , URI)
+        self.assertIsInstance(URI('http://user@www.example.com:1234/path0/path1?query#fragment') + 'hello', URI)
+        self.assertIsInstance(URI('http://user@www.example.com:1234/path0/path1?query#fragment') * 2, URI)
+        self.assertIsInstance(2 * URI('http://user@www.example.com:1234/path0/path1?query#fragment'), URI) # rmul
+        self.assertIsInstance(URI('http://user@www.example.com:1234/path0/path1?query#fragment').join(['hello', 'world']) , URI)
         # stripping
         self.assertIsInstance(URI('http://user@www.example.com:1234/path0/path1?query#fragment').strip(), URI)
         self.assertIsInstance(URI('http://user@www.example.com:1234/path0/path1?query#fragment').lstrip(), URI)
@@ -176,7 +181,6 @@ class TestURI(unittest.TestCase):
         self.assertIsInstance(URI('http://user@www.example.com:1234/path0/path1?query#fragment').lower(), URI)
         self.assertIsInstance(URI('http://user@www.example.com:1234/path0/path1?query#fragment').upper(), URI)
         # formatting
-        self.assertIsInstance(URI('http://user@www.example.com:1234/{}/path1?{}#fragment').format('hello', 'world'), URI)
         self.assertIsInstance(URI('http://user@www.example.com:1234/%s/path1?%s#fragment') % ('hello', 'world'), URI)
         self.assertIsInstance(URI('http://user@www.example.com:1234/path0/path1?query#fragment').replace('path0', 'pathX'), URI)
 
diff --git a/test/utils/test_uuid.py b/test/utils/test_uuid.py
index 49176d4..8f519d9 100644
--- a/test/utils/test_uuid.py
+++ b/test/utils/test_uuid.py
@@ -1,9 +1,4 @@
-"""
 
-Part of the tagit test suite.
-A copy of the license is provided with the project.
-Author: Matthias Baumgartner, 2022
-"""
 # imports
 import os
 import re
@@ -83,6 +78,20 @@ class TestUCID(unittest.TestCase):
     def test_from_path(self):
         self.assertEqual(UCID.from_path(self._path), self._checksum)
 
+    def test_from_buffer(self):
+        with open(self._path, 'rb') as ifile:
+            self.assertEqual(UCID.from_buffer(ifile), self._checksum)
+        with open(self._path) as ifile:
+            self.assertEqual(UCID.from_buffer(ifile), self._checksum)
+
+    def test_from_bytes(self):
+        with open(self._path, 'rb') as ifile:
+            self.assertEqual(UCID.from_bytes(ifile.read()), self._checksum)
+
+    def test_from_dict(self):
+        self.assertEqual(UCID.from_dict({'hello': 'world', 'foo': 1234, 'bar': False}),
+            '8d2544395a0d2827e3d9ce8cd619d5e3f801e8126bf3f93ee5abd38158959585')
+
 
 ## main ##