Feature support in the schema

* Type annotations
* Feature type
* Moved from_string from Schema to its own file/function
* Root predicate has a valid (not-None) range
* ROOT_... export in schema.types
* Empty as the default Schema constructor
* Schema loads some additional default symbols
* _Type instances compare along class hierarchy

7 files changed, 1578 insertions, 382 deletions

diff --git a/bsfs/schema/__init__.py b/bsfs/schema/__init__.py
index ad4d456..dc24313 100644
--- a/bsfs/schema/__init__.py
+++ b/bsfs/schema/__init__.py
@@ -9,7 +9,8 @@ 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 # FIXME: _Vertex
 
 # exports
 __all__: typing.Sequence[str] = (
@@ -17,6 +18,8 @@ __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..1c4c807 100644
--- a/bsfs/schema/schema.py
+++ b/bsfs/schema/schema.py
@@ -51,11 +51,13 @@ 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 +65,36 @@ 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_NUMBER)
+        predicates.add(types.ROOT_FEATURE)
+
         # 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 +228,7 @@ 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 +310,4 @@ 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)
-
 ## EOF ##
diff --git a/bsfs/schema/serialize.py b/bsfs/schema/serialize.py
new file mode 100644
index 0000000..1222aa6
--- /dev/null
+++ b/bsfs/schema/serialize.py
@@ -0,0 +1,143 @@
+"""
+
+Part of the BlackStar filesystem (bsfs) module.
+A copy of the license is provided with the project.
+Author: Matthias Baumgartner, 2022
+"""
+# standard imports
+from collections import abc
+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 _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)
+        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
+    literals = set(_fetch_hierarchically(types.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
+    # FIXME: type annotation
+    def _fetch_value(subject: URI, predicate: rdflib.URIRef, value_factory) -> 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
+        elif len(values) == 1:
+            return value_factory(values[0])
+        else:
+            raise errors.ConsistencyError(f'{subject} has multiple values for predicate {str(predicate)}, expected zero or one')
+
+    def _build_predicate(uri, parent, **annotations):
+        """Predicate factory."""
+        # break out on root feature type
+        if uri == types.ROOT_FEATURE.uri:
+            return types.ROOT_FEATURE
+        # 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}')
+        elif 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}')
+        elif 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)
+        # 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.get_child(URI(uri), domain=dom, range=rng, unique=unique,
+                dtype=dtype, dimension=dimension, distance=distance, **annotations)
+        # handle non-feature predicate
+        return parent.get_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) -> str:
+    """
+    """
+    raise NotImplementedError()
+
+## EOF ##
diff --git a/bsfs/schema/types.py b/bsfs/schema/types.py
index 54a7e99..e737263 100644
--- a/bsfs/schema/types.py
+++ b/bsfs/schema/types.py
@@ -8,6 +8,7 @@ Author: Matthias Baumgartner, 2022
 import typing
 
 # bsfs imports
+from bsfs.namespace import ns
 from bsfs.utils import errors, URI, typename
 
 # exports
@@ -15,6 +16,7 @@ __all__: typing.Sequence[str] = (
     'Literal',
     'Node',
     'Predicate',
+    'Feature',
     )
 
 
@@ -99,9 +101,11 @@ class _Type():
             self,
             uri: URI,
             parent: typing.Optional['_Type'] = None,
+            **annotations: typing.Any,
             ):
         self.uri = uri
         self.parent = parent
+        self.annotations = annotations
 
     def parents(self) -> typing.Generator['_Type', None, None]:
         """Generate a list of parent nodes."""
@@ -110,9 +114,17 @@ class _Type():
             yield curr
             curr = curr.parent
 
-    def get_child(self, uri: URI, **kwargs):
+    def get_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})'
@@ -138,7 +150,7 @@ 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(self)):
             return NotImplemented
         if self.uri == other.uri: # equivalence
             return False
@@ -151,7 +163,7 @@ 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(self)):
             return NotImplemented
         if self.uri == other.uri: # equivalence
             return True
@@ -164,7 +176,7 @@ 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(self)):
             return NotImplemented
         if self.uri == other.uri: # equivalence
             return False
@@ -177,7 +189,7 @@ 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(self)):
             return NotImplemented
         if self.uri == other.uri: # equivalence
             return True
@@ -191,30 +203,33 @@ class _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):
     """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):
     """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 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
@@ -223,25 +238,26 @@ class Predicate(_Type):
             self,
             # Type members
             uri: URI,
-            parent: typing.Optional['Predicate'],
+            parent: '_PredicateBase',
             # 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) \
@@ -264,13 +280,132 @@ 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().get_child(
+            uri=uri,
+            domain=domain,
+            range=range,
+            unique=unique,
+            **kwargs
+            )
+
+
+class Feature(Predicate):
+    """Feature base 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: Predicate,
+            # Predicate members
+            domain: Node,
+            range: Literal,
+            unique: bool,
+            # Feature members
+            dimension: int,
+            dtype: URI,
+            distance: URI,
+            **kwargs,
+            ):
+        super().__init__(uri, parent, domain, range, unique, **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 get_child(
+            self,
+            uri: URI,
+            domain: typing.Optional[Node] = None,
+            range: typing.Optional[Literal] = None, # pylint: disable=redefined-builtin
+            unique: typing.Optional[bool] = None,
+            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().get_child(
+            uri=uri,
+            domain=domain,
+            range=range,
+            unique=unique,
+            dimension=dimension,
+            dtype=dtype,
+            distance=distance,
+            **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_NUMBER = Literal(
+    uri=ns.bsfs.Number,
+    parent=ROOT_LITERAL,
+    )
+
+# essential predicates
+ROOT_PREDICATE = Predicate(
+    uri=ns.bsfs.Predicate,
+    parent=None,
+    domain=ROOT_NODE,
+    range=ROOT_VERTEX,
+    unique=False,
+    )
+
+ROOT_FEATURE = Feature(
+    uri=ns.bsfs.Feature,
+    parent=ROOT_PREDICATE,
+    domain=ROOT_NODE,
+    range=ROOT_LITERAL,
+    unique=False,
+    dimension=1,
+    dtype=ns.bsfs.f16,
+    distance=ns.bsfs.euclidean,
+    )
+
 ## EOF ##
diff --git a/test/schema/test_schema.py b/test/schema/test_schema.py
index 888cdca..1b45db0 100644
--- a/test/schema/test_schema.py
+++ b/test/schema/test_schema.py
@@ -10,7 +10,7 @@ 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
@@ -55,7 +55,7 @@ class TestSchema(unittest.TestCase):
 
             '''
         # nodes
-        self.n_root = types.Node(ns.bsfs.Node, None)
+        self.n_root = types.ROOT_NODE
         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))
@@ -63,24 +63,32 @@ class TestSchema(unittest.TestCase):
         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_root = types.ROOT_LITERAL
+        self.l_number = types.ROOT_NUMBER
         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_integer = types.Literal(ns.xsd.integer, self.l_number)
         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.literals = [self.l_root, self.l_number, self.l_string, self.l_integer, self.l_unused]
 
         # predicates
-        self.p_root = types.Predicate(ns.bsfs.Predicate, None, types.Node(ns.bsfs.Node, None), None, False)
+        self.p_root = types.ROOT_PREDICATE
+        self.f_root = types.ROOT_FEATURE
         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.predicates = [self.p_root, self.p_tag, self.p_group, self.p_comment]
+        self.predicates = [self.p_root, self.f_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.assertSetEqual(set(schema.predicates()), {self.p_root, self.f_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.assertSetEqual(set(schema.predicates()), set(self.predicates))
 
         # predicates, nodes, and literals are respected
@@ -101,21 +109,21 @@ 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})
         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})
         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})
         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})
         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, self.f_root})
         schema = Schema([self.p_group], self.nodes, self.literals)
-        self.assertSetEqual(set(schema.predicates()), {self.p_root, self.p_tag, self.p_group})
+        self.assertSetEqual(set(schema.predicates()), {self.p_root, self.f_root, self.p_tag, self.p_group})
         schema = Schema([self.p_group, self.p_comment], self.nodes, self.literals)
         self.assertSetEqual(set(schema.predicates()), set(self.predicates))
 
@@ -153,21 +161,28 @@ 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.bsfs.Number]
+        p = [ns.bsfs.Feature, 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]
-        p = [ns.bse.comment, ns.bse.group, ns.bse.tag, ns.bsfs.Predicate]
+        l = [ns.bsfs.Literal, ns.bsfs.Number, ns.xsd.boolean, ns.xsd.integer, ns.xsd.string]
+        p = [ns.bse.comment, ns.bse.group, ns.bse.tag, ns.bsfs.Feature, ns.bsfs.Predicate]
         self.assertEqual(repr(Schema(self.predicates, self.nodes, self.literals)), f'Schema({n}, {l}, {p})')
 
     def test_equality(self):
@@ -258,11 +273,11 @@ 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})))
 
@@ -280,11 +295,11 @@ 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})))
 
@@ -351,26 +366,26 @@ 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 +562,6 @@ 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..7392cc0
--- /dev/null
+++ b/test/schema/test_serialize.py
@@ -0,0 +1,1007 @@
+"""
+
+Part of the tagit test suite.
+A copy of the license is provided with the project.
+Author: Matthias Baumgartner, 2022
+"""
+# imports
+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 ##
+
+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: <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 .
+            ''')
+
+
+    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: <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 .
+            ''')
+
+        # 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: <http://bsfs.ai/schema/>
+
+            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.get_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: <http://bsfs.ai/schema/>
+            prefix bse: <http://bsfs.ai/schema/Entity#>
+
+            bsfs:unused rdfs:subClassOf bsfs:Node . # unused symbol
+            '''))
+
+        # a node can have multiple children
+        n_ent = types.ROOT_NODE.get_child(ns.bsfs.Entity)
+        n_tag = types.ROOT_NODE.get_child(ns.bsfs.Tag)
+        n_doc = n_ent.get_child(ns.bsfs.Document)
+        n_image = n_ent.get_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: <http://bsfs.ai/schema/>
+            prefix bse: <http://bsfs.ai/schema/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.get_child(ns.bsfs.Entity)
+        l_string = types.ROOT_LITERAL.get_child(ns.xsd.string)
+        p_filename = types.ROOT_PREDICATE.get_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: <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 .
+            '''))
+
+        # 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: <http://bsfs.ai/schema/>
+
+            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: <http://bsfs.ai/schema/>
+
+            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: <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 ; # 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: <http://bsfs.ai/schema/>
+
+            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.get_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: <http://bsfs.ai/schema/>
+            prefix bse: <http://bsfs.ai/schema/Entity#>
+
+            xsd:unused rdfs:subClassOf bsfs:Literal . # unused symbol
+            '''))
+
+        # a literal can have multiple children
+        l_string = types.ROOT_LITERAL.get_child(ns.xsd.string)
+        l_integer = types.ROOT_LITERAL.get_child(ns.xsd.integer)
+        l_unsigned = l_integer.get_child(ns.xsd.unsigned)
+        l_signed = l_integer.get_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: <http://bsfs.ai/schema/>
+            prefix bse: <http://bsfs.ai/schema/Entity#>
+
+            # literals inherit from same parent
+            xsd:string rdfs:subClassOf bsfs:Literal .
+            xsd:integer rdfs:subClassOf bsfs:Literal .
+
+            # 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.get_child(ns.bsfs.Entity)
+        l_string = types.ROOT_LITERAL.get_child(ns.xsd.string)
+        p_filename = types.ROOT_PREDICATE.get_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: <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 .
+            '''))
+
+        # 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: <http://bsfs.ai/schema/>
+
+            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: <http://bsfs.ai/schema/>
+
+            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: <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 ; # 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: <http://bsfs.ai/schema/>
+            prefix bse: <http://bsfs.ai/schema/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: <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 ; # 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: <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 ; # 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: <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 ; # invalid symbol
+                bsfs:unique "false"^^xsd:boolean .
+            ''')
+
+        # additional predicates can be defined
+        n_ent = types.ROOT_NODE.get_child(ns.bsfs.Entity)
+        l_string = types.ROOT_LITERAL.get_child(ns.xsd.string)
+        p_comment = types.ROOT_PREDICATE.get_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: <http://bsfs.ai/schema/>
+            prefix bse: <http://bsfs.ai/schema/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.get_child(ns.bsfs.Entity)
+        l_string = types.ROOT_LITERAL.get_child(ns.xsd.string)
+        p_annotation = types.ROOT_PREDICATE.get_child(ns.bsfs.Annotation, domain=n_ent, range=l_string)
+        p_comment = p_annotation.get_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: <http://bsfs.ai/schema/>
+            prefix bse: <http://bsfs.ai/schema/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.get_child(ns.bsfs.Entity)
+        l_string = types.ROOT_LITERAL.get_child(ns.xsd.string)
+        p_annotation = types.ROOT_PREDICATE.get_child(ns.bsfs.Annotation, domain=n_ent)
+        p_comment = p_annotation.get_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: <http://bsfs.ai/schema/>
+            prefix bse: <http://bsfs.ai/schema/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.get_child(ns.bsfs.Entity)
+        p_foo = types.ROOT_PREDICATE.get_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: <http://bsfs.ai/schema/>
+            prefix bse: <http://bsfs.ai/schema/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.get_child(ns.bsfs.Entity)
+        n_image = n_ent.get_child(ns.bsfs.Image)
+        p_foo = types.ROOT_PREDICATE.get_child(ns.bse.foo, domain=types.ROOT_NODE)
+        p_bar = p_foo.get_child(ns.bse.bar, domain=n_ent)
+        p_foobar = p_bar.get_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: <http://bsfs.ai/schema/>
+            prefix bse: <http://bsfs.ai/schema/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: <http://bsfs.ai/schema/>
+            prefix bse: <http://bsfs.ai/schema/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.get_child(ns.bsfs.Entity)
+        n_image = n_ent.get_child(ns.bsfs.Image)
+        p_foo = types.ROOT_PREDICATE.get_child(ns.bse.foo, range=types.ROOT_NODE)
+        p_bar = p_foo.get_child(ns.bse.bar, range=n_ent)
+        p_foobar = p_bar.get_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: <http://bsfs.ai/schema/>
+            prefix bse: <http://bsfs.ai/schema/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: <http://bsfs.ai/schema/>
+            prefix bse: <http://bsfs.ai/schema/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: <http://bsfs.ai/schema/>
+            prefix bse: <http://bsfs.ai/schema/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: <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 . # 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: <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 . # 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: <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 . # 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: <http://bsfs.ai/schema/>
+            prefix bse: <http://bsfs.ai/schema/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: <http://bsfs.ai/schema/>
+            prefix bse: <http://bsfs.ai/schema/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):
+        # 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: <http://bsfs.ai/schema/>
+            prefix bse: <http://bsfs.ai/schema/Entity#>
+
+            bsfs:Feature rdfs:subClassOf bsfs:Predicate .
+            bsfs:array rdfs:subClassOf bsfs:Literal .
+
+            bse:colors rdfs:subClassOf bsfs:Feature ;
+                rdfs:domain bsfs:Entity ; # undefined symbol
+                rdfs:range bsfs:array ;
+                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: <http://bsfs.ai/schema/>
+            prefix bse: <http://bsfs.ai/schema/Entity#>
+
+            bsfs:Feature rdfs:subClassOf bsfs:Predicate .
+            bsfs:Entity rdfs:subClassOf bsfs:Literal .
+            bsfs:array rdfs:subClassOf bsfs:Literal .
+
+            bse:colors rdfs:subClassOf bsfs:Feature ;
+                rdfs:domain bsfs:Entity ; # literal instead of node
+                rdfs:range bsfs:array ;
+                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: <http://bsfs.ai/schema/>
+            prefix bse: <http://bsfs.ai/schema/Entity#>
+
+            bsfs:Feature rdfs:subClassOf bsfs:Predicate .
+            bsfs:Entity rdfs:subClassOf bsfs:Node .
+
+            bse:colors rdfs:subClassOf bsfs:Feature ;
+                rdfs:domain bsfs:Entity ;
+                rdfs:range bsfs:array ; # 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: <http://bsfs.ai/schema/>
+            prefix bse: <http://bsfs.ai/schema/Entity#>
+
+            bsfs:Feature rdfs:subClassOf bsfs:Predicate .
+            bsfs:Entity rdfs:subClassOf bsfs:Node .
+
+            bse:colors rdfs:subClassOf bsfs:Feature ;
+                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: <http://bsfs.ai/schema/>
+            prefix bse: <http://bsfs.ai/schema/Entity#>
+
+            bsfs:Feature rdfs:subClassOf bsfs:Predicate .
+            bsfs:Entity rdfs:subClassOf bsfs:Node .
+
+            bse:colors rdfs:subClassOf bsfs:Feature ;
+                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.get_child(ns.bsfs.Entity)
+        l_array = types.ROOT_LITERAL.get_child(ns.bsfs.array)
+        p_comment = types.ROOT_FEATURE.get_child(ns.bse.colors, domain=n_ent, range=l_array, 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: <http://bsfs.ai/schema/>
+            prefix bse: <http://bsfs.ai/schema/Entity#>
+
+            bsfs:Feature rdfs:subClassOf bsfs:Predicate .
+            bsfs:Entity rdfs:subClassOf bsfs:Node .
+            bsfs:array rdfs:subClassOf bsfs:Literal .
+
+            bse:colors rdfs:subClassOf bsfs:Feature ;
+                rdfs:domain bsfs:Entity ;
+                rdfs:range bsfs:array ;
+                bsfs:unique "false"^^xsd:boolean .
+            '''))
+
+        # features inherit properties from parents
+        n_ent = types.ROOT_NODE.get_child(ns.bsfs.Entity)
+        l_array = types.ROOT_LITERAL.get_child(ns.bsfs.array)
+        l_string = types.ROOT_LITERAL.get_child(ns.xsd.string)
+        p_annotation = types.ROOT_FEATURE.get_child(ns.bsfs.Annotation, domain=n_ent, range=l_array,
+            dimension=1234, dtype=ns.xsd.string)
+        p_comment = p_annotation.get_child(ns.bse.colors, 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: <http://bsfs.ai/schema/>
+            prefix bse: <http://bsfs.ai/schema/Entity#>
+
+            bsfs:Feature rdfs:subClassOf bsfs:Predicate .
+            bsfs:Entity rdfs:subClassOf bsfs:Node .
+            bsfs:array rdfs:subClassOf bsfs:Literal .
+
+            bsfs:Annotation rdfs:subClassOf bsfs:Feature ; # inherits defaults from bsfs:Feature
+                rdfs:domain bsfs:Entity ;
+                rdfs:range bsfs:array ;
+                bsfs:dimension "1234"^^xsd:integer ;
+                bsfs:dtype xsd:string .
+
+            bse:colors rdfs:subClassOf bsfs:Annotation ; # inherits domain/range/etc. from bsfs:Annotation
+                bsfs:unique "true"^^xsd:boolean . # overwrites bsfs:Predicate
+            '''))
+
+        # feature definition can be split across multiple statements.
+        # statements can be repeated
+        n_ent = types.ROOT_NODE.get_child(ns.bsfs.Entity)
+        p_foo = types.ROOT_FEATURE.get_child(ns.bse.foo, domain=n_ent, unique=True,
+                dimension=1234, dtype=ns.bsfs.f32)
+        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: <http://bsfs.ai/schema/>
+            prefix bse: <http://bsfs.ai/schema/Entity#>
+
+            bsfs:Feature rdfs:subClassOf bsfs:Predicate .
+            bsfs:Entity rdfs:subClassOf bsfs:Node .
+
+            bse:foo rdfs:subClassOf bsfs:Feature ;
+                bsfs:unique "true"^^xsd:boolean ;
+                bsfs:dimension "1234"^^xsd:integer .
+
+            bse:foo rdfs:subClassOf bsfs:Feature ;
+                rdfs:domain bsfs:Entity ;
+                bsfs:dtype bsfs: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: <http://bsfs.ai/schema/>
+            prefix bse: <http://bsfs.ai/schema/Entity#>
+
+            bsfs:Feature rdfs:subClassOf bsfs:Predicate .
+            bsfs:Annotation rdfs:subClassOf bsfs:Feature .
+            bsfs:Entity rdfs:subClassOf bsfs:Node .
+
+            bse:foo rdfs:subClassOf bsfs:Feature ;
+                rdfs:domain bsfs:Node ;
+                bsfs:unique "false"^^xsd:boolean .
+
+            bse:foo rdfs:subClassOf bsfs:Annotation ;
+                rdfs:domain bsfs:Node ;
+                bsfs:unique "false"^^xsd:boolean .
+
+            ''')
+        # cannot assign multiple conflicting domains 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: <http://bsfs.ai/schema/>
+            prefix bse: <http://bsfs.ai/schema/Entity#>
+
+            bsfs:Feature rdfs:subClassOf bsfs:Predicate .
+            bsfs:Entity rdfs:subClassOf bsfs:Node .
+
+            bse:foo rdfs:subClassOf bsfs:Feature ;
+                rdfs:domain bsfs:Node ;
+                bsfs:unique "false"^^xsd:boolean .
+
+            bse:foo rdfs:subClassOf bsfs:Feature ;
+                rdfs:domain bsfs:Entity . # conflicting domain
+            ''')
+        # cannot assign multiple conflicting ranges 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: <http://bsfs.ai/schema/>
+            prefix bse: <http://bsfs.ai/schema/Entity#>
+
+            bsfs:Feature rdfs:subClassOf bsfs:Predicate .
+            bsfs:Entity rdfs:subClassOf bsfs:Node .
+            bsfs:array rdfs:subClassOf bsfs:Literal .
+            bsfs:large_array rdfs:subClassOf bsfs:array .
+            bsfs:small_array rdfs:subClassOf bsfs:array .
+
+            bse:foo rdfs:subClassOf bsfs:Feature ;
+                rdfs:domain bsfs:Node ;
+                rdfs:range bsfs:large_array ;
+                bsfs:unique "false"^^xsd:boolean .
+
+            bse:foo rdfs:subClassOf bsfs:Feature ;
+                rdfs:range bsfs:small_array . # conflicting range
+            ''')
+        # cannot assign multiple conflicting uniques 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: <http://bsfs.ai/schema/>
+            prefix bse: <http://bsfs.ai/schema/Entity#>
+
+            bsfs:Feature rdfs:subClassOf bsfs:Predicate .
+            bsfs:Entity rdfs:subClassOf bsfs:Node .
+
+            bse:foo rdfs:subClassOf bsfs:Feature ;
+                rdfs:domain bsfs:Node ;
+                rdfs:range bsfs:Node ;
+                bsfs:unique "false"^^xsd:boolean .
+
+            bse:foo rdfs:subClassOf bsfs:Feature ;
+                bsfs:unique "true"^^xsd:boolean . # conflicting unique
+            ''')
+        # 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: <http://bsfs.ai/schema/>
+            prefix bse: <http://bsfs.ai/schema/Entity#>
+
+            bsfs:Feature rdfs:subClassOf bsfs:Predicate .
+            bsfs:Entity rdfs:subClassOf bsfs:Node .
+
+            bse:foo rdfs:subClassOf bsfs:Feature ;
+                rdfs:domain bsfs:Node ;
+                bsfs:dimension "1234"^^xsd:integer .
+
+            bse:foo rdfs:subClassOf bsfs: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: <http://bsfs.ai/schema/>
+            prefix bse: <http://bsfs.ai/schema/Entity#>
+
+            bsfs:Feature rdfs:subClassOf bsfs:Predicate .
+            bsfs:Entity rdfs:subClassOf bsfs:Node .
+
+            bse:foo rdfs:subClassOf bsfs:Feature ;
+                rdfs:domain bsfs:Node ;
+                bsfs:dtype bsfs:f32 .
+
+            bse:foo rdfs:subClassOf bsfs:Feature ;
+                bsfs:dtype bsfs: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: <http://bsfs.ai/schema/>
+            prefix bse: <http://bsfs.ai/schema/Entity#>
+
+            bsfs:Feature rdfs:subClassOf bsfs:Predicate .
+            bsfs:Entity rdfs:subClassOf bsfs:Node .
+
+            bse:foo rdfs:subClassOf bsfs:Feature ;
+                rdfs:domain bsfs:Node ;
+                bsfs:distance bsfs:euclidean .
+
+            bse:foo rdfs:subClassOf bsfs: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: <http://bsfs.ai/schema/>
+            prefix bse: <http://bsfs.ai/schema/Entity#>
+
+            bsfs:Feature rdfs:subClassOf bsfs:Predicate .
+            bse:colors rdfs:subClassOf bsfs:Feature ;
+                bsfs:dimension "1234"^^xsd:integer .
+
+            ''').predicate(ns.bse.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: <http://bsfs.ai/schema/>
+            prefix bse: <http://bsfs.ai/schema/Entity#>
+
+            bsfs:Feature rdfs:subClassOf bsfs:Predicate .
+            bse:colors rdfs:subClassOf bsfs:Feature ;
+                bsfs:dimension "1234"^^xsd:integer ;
+                rdfs:label "hello world"^^xsd:string ;
+                bsfs:foo "1234"^^xsd:integer .
+
+            ''').predicate(ns.bse.colors).annotations, {
+            ns.rdfs.label: 'hello world',
+            ns.bsfs.foo: 1234,
+            })
+
+
+    def test_integration(self):
+        # nodes
+        n_ent = types.ROOT_NODE.get_child(ns.bsfs.Entity)
+        n_tag = types.ROOT_NODE.get_child(ns.bsfs.Tag)
+        n_image = n_ent.get_child(ns.bsfs.Image)
+        # literals
+        l_string = types.ROOT_LITERAL.get_child(ns.xsd.string)
+        l_array = types.ROOT_LITERAL.get_child(ns.bsfs.array)
+        l_integer = types.ROOT_LITERAL.get_child(ns.xsd.integer)
+        l_boolean = types.ROOT_LITERAL.get_child(ns.xsd.boolean)
+        # predicates
+        p_annotation = types.ROOT_PREDICATE.get_child(ns.bsfs.Annotation)
+        p_tag = types.ROOT_PREDICATE.get_child(ns.bse.tag, domain=n_ent, range=n_tag)
+        p_group = p_tag.get_child(ns.bse.group, domain=n_image, unique=True)
+        p_comment = p_annotation.get_child(ns.bse.comment, range=l_string)
+        # features
+        f_colors = types.ROOT_FEATURE.get_child(URI('http://bsfs.ai/schema/Feature/colors_spatial'),
+            domain=n_ent, range=l_array, unique=True, dtype=ns.bsfs.f16, distance=ns.bsfs.euclidean)
+        f_colors1234 = f_colors.get_child(URI('http://bsfs.ai/schema/Feature/colors_spatial#1234'), dimension=1024)
+        f_colors4321 = f_colors.get_child(URI('http://bsfs.ai/schema/Feature/colors_spatial#4321'), dimension=2048)
+        # schema
+        ref = Schema(
+            {p_annotation, p_tag, p_group, p_comment, f_colors, f_colors1234, f_colors4321},
+            {n_ent, n_tag, n_image},
+            {l_string, l_integer, l_boolean})
+        # 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: <http://bsfs.ai/schema/>
+            prefix bse: <http://bsfs.ai/schema/Entity#>
+
+            # 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 .
+            bsfs:array rdfs:subClassOf bsfs:Literal .
+            xsd:integer rdfs:subClassOf bsfs:Literal .
+            xsd:boolean rdfs:subClassOf bsfs:Literal .
+
+            # abstract predicates
+            bsfs:Annotation rdfs:subClassOf bsfs:Predicate ;
+                rdfs:label "node annotation"^^xsd:string .
+            bsfs:Feature rdfs:subClassOf bsfs:Predicate .
+
+            # feature instances
+            <http://bsfs.ai/schema/Feature/colors_spatial> rdfs:subClassOf bsfs:Feature ;
+                rdfs:domain bsfs:Entity ;
+                rdfs:range bsfs:array ;
+                bsfs:unique "true"^^xsd:boolean ;
+                bsfs:dtype bsfs: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 .
+
+            <http://bsfs.ai/schema/Feature/colors_spatial#1234> rdfs:subClassOf <http://bsfs.ai/schema/Feature/colors_spatial> ;
+                bsfs:dimension "1024"^^xsd:integer ;
+                rdfs:label "Main colors spatial instance"^^xsd:string .
+
+            <http://bsfs.ai/schema/Feature/colors_spatial#4321> rdfs:subClassOf <http://bsfs.ai/schema/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.predicate(URI('http://bsfs.ai/schema/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.predicate(URI('http://bsfs.ai/schema/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'})
+
+
+
+class TestToString(unittest.TestCase):
+    def test_stub(self):
+        raise NotImplementedError()
+
+
+## main ##
+
+if __name__ == '__main__':
+    unittest.main()
+
+## EOF ##
diff --git a/test/schema/test_types.py b/test/schema/test_types.py
index 4a49e6e..af47f0d 100644
--- a/test/schema/test_types.py
+++ b/test/schema/test_types.py
@@ -10,15 +10,17 @@ 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 ##
 
 class TestType(unittest.TestCase):
+
     def test_parents(self):
         # create some types
         fst = _Type('First')
@@ -31,7 +33,25 @@ 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 get_child
+        self.assertDictEqual(_Type('First', foo='bar').get_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)')
@@ -59,6 +79,9 @@ class TestType(unittest.TestCase):
         # type persists
         class Foo(_Type): pass
         self.assertEqual(Foo('First').get_child('Second'), Foo('Second', Foo('First')))
+        # annotations are respected
+        self.assertDictEqual(_Type('First', foo='bar').get_child('Second', bar='foo').annotations, {
+            'bar': 'foo'})
 
     def test_equality(self):
         # equality depends on uri
@@ -76,6 +99,13 @@ 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.
@@ -109,25 +139,40 @@ class TestType(unittest.TestCase):
         self.assertFalse(bike > bicycle)
         self.assertFalse(bike >= bicycle)
         self.assertFalse(bike == bicycle)
+
+        # can compare types along the class hierarchy
         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.assertTrue(foo < bike)
+        self.assertTrue(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.assertTrue(bike > foo)
+        self.assertTrue(bike >= foo)
+        # cannot compare unrelated classes
+        class Bar(_Type): pass
+        bar = Bar('Bar', bike)
+        self.assertRaises(TypeError, operator.lt, foo, bar)
+        self.assertRaises(TypeError, operator.le, foo, bar)
+        self.assertRaises(TypeError, operator.gt, foo, bar)
+        self.assertRaises(TypeError, operator.ge, foo, bar)
+        # goes both ways
+        self.assertRaises(TypeError, operator.lt, bar, foo)
+        self.assertRaises(TypeError, operator.le, bar, foo)
+        self.assertRaises(TypeError, operator.gt, bar, foo)
+        self.assertRaises(TypeError, operator.ge, 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), _Type(ns.bsfs.Foo, None), True)
@@ -138,54 +183,52 @@ 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):
         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,
             )
 
+        # get_child returns Predicate
+        self.assertIsInstance(tag.get_child(ns.bse.foo), Predicate)
         # uri is respected
         self.assertEqual(ns.bse.foo, tag.get_child(ns.bse.foo).uri)
         # domain is respected
@@ -198,10 +241,17 @@ class TestPredicate(unittest.TestCase):
         self.assertEqual(n_tag, tag.get_child(ns.bse.foo, range=None).range)
         # unique is respected
         self.assertTrue(tag.get_child(ns.bse.foo, unique=True).unique)
+        # annotations are respected
+        self.assertDictEqual(tag.get_child(ns.bse.foo, foo='bar', bar=123).annotations, {
+            'foo': 'bar',
+            'bar': 123,
+            })
 
         # domain is inherited from parent
+        self.assertEqual(n_root, root.get_child(ns.bse.foo).domain)
         self.assertEqual(n_ent, tag.get_child(ns.bse.foo).domain)
         # range is inherited from parent
+        self.assertEqual(ROOT_VERTEX, root.get_child(ns.bse.foo).range)
         self.assertEqual(n_tag, tag.get_child(ns.bse.foo).range)
         # uniqueness is inherited from parent
         self.assertFalse(tag.get_child(ns.bse.foo).unique)
@@ -209,11 +259,118 @@ class TestPredicate(unittest.TestCase):
         # 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)
         # 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(TypeError, tag.get_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.get_child(ns.bse.foo, range=n_root).range)
+        self.assertEqual(l_root, root.get_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, n_root, l_root, False,
+            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))
+        l_array = Literal(ns.bsfs.array, Literal(ns.bsfs.Literal, None))
+        colors = Feature(
+            uri=ns.bse.colors,
+            parent=ROOT_FEATURE,
+            domain=n_ent,
+            range=l_array,
+            unique=False,
+            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, n_ent, l_array, False, 1234, ns.bsfs.float, ns.bsfs.euclidean))
+        self.assertEqual(hash(colors), hash(Feature(ns.bse.colors, ROOT_FEATURE, n_ent, l_array, False, 1234, ns.bsfs.float, ns.bsfs.euclidean)))
+        # equality respects dimension
+        self.assertNotEqual(colors, Feature(ns.bse.colors, ROOT_FEATURE, n_ent, l_array, False, 4321, ns.bsfs.float, ns.bsfs.euclidean))
+        self.assertNotEqual(hash(colors), hash(Feature(ns.bse.colors, ROOT_FEATURE, n_ent, l_array, False, 4321, ns.bsfs.float, ns.bsfs.euclidean)))
+        # equality respects dtype
+        self.assertNotEqual(colors, Feature(ns.bse.colors, ROOT_FEATURE, n_ent, l_array, False, 1234, ns.bsfs.integer, ns.bsfs.euclidean))
+        self.assertNotEqual(hash(colors), hash(Feature(ns.bse.colors, ROOT_FEATURE, n_ent, l_array, False, 1234, ns.bsfs.integer, ns.bsfs.euclidean)))
+        # equality respects distance
+        self.assertNotEqual(colors, Feature(ns.bse.colors, ROOT_FEATURE, n_ent, l_array, False, 1234, ns.bsfs.float, ns.bsfs.cosine))
+        self.assertNotEqual(hash(colors), hash(Feature(ns.bse.colors, ROOT_FEATURE, n_ent, l_array, False, 1234, ns.bsfs.float, ns.bsfs.cosine)))
+
+    def test_get_child(self):
+        n_root = Node(ns.bsfs.Node, None)
+        n_ent = Node(ns.bsfs.Entity, n_root)
+        l_root = Literal(ns.bsfs.Literal, None)
+        l_array = Literal(ns.bsfs.array, l_root)
+        colors = Feature(
+            uri=ns.bse.colors,
+            parent=ROOT_FEATURE,
+            domain=n_ent,
+            range=l_array,
+            unique=False,
+            dimension=1234,
+            dtype=ns.bsfs.float,
+            distance=ns.bsfs.euclidean,
+            )
+
+        # get_child returns Feature
+        self.assertIsInstance(colors.get_child(ns.bse.foo), Feature)
+        # uri is respected
+        self.assertEqual(ns.bse.foo, colors.get_child(ns.bse.foo).uri)
+        # domain is respected
+        dom = Node(ns.bsfs.Image, n_ent)
+        self.assertEqual(dom, colors.get_child(ns.bse.foo, domain=dom).domain)
+        # range is respected
+        rng = Literal(ns.bse.foo, l_array)
+        self.assertEqual(rng, colors.get_child(ns.bse.foo, range=rng).range)
+        # cannot set range to None
+        self.assertEqual(l_array, colors.get_child(ns.bse.foo, range=None).range)
+        # unique is respected
+        self.assertTrue(colors.get_child(ns.bse.foo, unique=True).unique)
+        # dimension is respected
+        self.assertEqual(4321, colors.get_child(ns.bse.foo, dimension=4321).dimension)
+        # dtype is respected
+        self.assertEqual(ns.bsfs.integer, colors.get_child(ns.bse.foo, dtype=ns.bsfs.integer).dtype)
+        # distance is respected
+        self.assertEqual(ns.bsfs.cosine, colors.get_child(ns.bse.foo, distance=ns.bsfs.cosine).distance)
+        # annotations are respected
+        self.assertDictEqual(colors.get_child(ns.bse.foo, foo='bar', bar=123).annotations, {
+            'foo': 'bar',
+            'bar': 123,
+            })
+
+        # domain is inherited from parent
+        self.assertEqual(n_root, ROOT_FEATURE.get_child(ns.bse.foo).domain)
+        self.assertEqual(n_ent, colors.get_child(ns.bse.foo).domain)
+        # range is inherited from parent
+        self.assertEqual(l_array, colors.get_child(ns.bse.foo).range)
+        # uniqueness is inherited from parent
+        self.assertFalse(colors.get_child(ns.bse.foo).unique)
+        # dimension is inherited from parent
+        self.assertEqual(1234, colors.get_child(ns.bse.foo).dimension)
+        # dtype is inherited from parent
+        self.assertEqual(ns.bsfs.float, colors.get_child(ns.bse.foo).dtype)
+        # distance is inherited from parent
+        self.assertEqual(ns.bsfs.euclidean, colors.get_child(ns.bse.foo).distance)
+
+        # domain must be subtype of parent's domain
+        self.assertRaises(errors.ConsistencyError, colors.get_child, ns.bse.foo, domain=n_root)
+        self.assertRaises(errors.ConsistencyError, colors.get_child, ns.bse.foo, domain=Node(ns.bsfs.Image, n_root))
+        # range must be subtype of parent's range
+        self.assertRaises(errors.ConsistencyError, colors.get_child, ns.bse.foo, range=Literal(ns.bsfs.Literal, None))
+        self.assertRaises(errors.ConsistencyError, colors.get_child, ns.bse.foo, range=Literal(ns.bsfs.foo, Literal(ns.bsfs.Literal, None)))
+        self.assertRaises(TypeError, colors.get_child, ns.bse.foo, range=Node(ns.bsfs.Tag, n_root))
 
 
 ## main ##
@@ -222,4 +379,3 @@ if __name__ == '__main__':
     unittest.main()
 
 ## EOF ##
-