Define top-level class core:UcoThing; test for top-levelness

This patch is known to fail CI due to the new test.  It is posted early
for definition-comment feedback.

References:
* #430

Signed-off-by: Alex Nelson <alexander.nelson@nist.gov>

ontology/uco/core/core.ttl

@@ -322,8 +322,9 @@ core:UcoObject
 		owl:Class ,
 		sh:NodeShape
 		;
+	rdfs:subClassOf core:UcoThing ;
 	rdfs:label "UcoObject"@en ;
-	rdfs:comment "A UCO object is a representation of a fundamental concept either directly inherent to the cyber domain or indirectly related to the cyber domain and necessary for contextually characterizing cyber domain concepts and relationships. Within the Unified Cyber Ontology (UCO) structure this is the base class acting as a consistent, unifying and interoperable foundation for all explicit and inter-relatable content objects."@en ;
+	rdfs:comment "A UCO object is a representation of a fundamental concept either directly inherent to the cyber domain or indirectly related to the cyber domain and necessary for contextually characterizing cyber domain concepts and relationships. Within the Unified Cyber Ontology (UCO) structure this is a base class acting as a consistent, unifying and interoperable foundation for all explicit and inter-relatable content objects."@en ;
 	owl:disjointWith core:Facet ;
 	sh:property
 		[
@@ -396,6 +397,17 @@ core:UcoObject
 	sh:targetClass core:UcoObject ;
 	.
 
+core:UcoThing
+	a
+		owl:Class ,
+		sh:NodeShape
+		;
+	rdfs:subClassOf owl:Thing ;
+	rdfs:label "UcoThing"@en ;
+	rdfs:comment "UcoThing is the top-level class within UCO."@en ;
+	sh:targetClass core:UcoThing ;
+	.
+
 core:confidence
 	a owl:DatatypeProperty ;
 	rdfs:label "confidence"@en ;

tests/test_uco_monolithic.py

@@ -17,11 +17,13 @@
 
 import pytest
 import rdflib.plugins.sparql.processor
-from rdflib import BNode, Graph, Literal, RDF, URIRef
+from rdflib import BNode, Graph, Literal, Namespace, RDF, URIRef
 from rdflib.term import Node
 
 IdentifiedNode = Union[BNode, URIRef]
 
+NS_UCO_CORE = Namespace("https://ontology.unifiedcyberontology.org/uco/core/")
+
 
 @pytest.fixture(scope="module")
 def graph() -> Generator[Graph, None, None]:
@@ -162,3 +164,32 @@ def test_semi_open_vocabulary_owl_shacl_alignment(graph: Graph) -> None:
             computed.add((test_case[0], test_case[1], shacl_tuple, rdfs_tuple))
 
     assert expected == computed
+
+
+def test_only_one_uco_class_is_owl_thing_direct_subclass(graph: Graph) -> None:
+    """
+    UCO expects all classes defined in UCO namespaces (i.e. excluding the "import review" ontologies) are subclasses of core:UcoThing.  Within OWL, absence of an rdfs:subClassOf statement implies being a subclass of owl:Thing.  Review UCO for any accidental omission of rdfs:subClassOf.
+    """
+    expected: Set[URIRef] = {NS_UCO_CORE.UcoThing}
+    computed: Set[URIRef] = set()
+    for result in graph.query("""\
+SELECT ?nClass
+WHERE {
+    {
+        ?nClass rdfs:subClassOf owl:Thing .
+    }
+    UNION
+    {
+        ?nClass a owl:Class .
+        FILTER NOT EXISTS {
+            ?nClass rdfs:subClassOf ?nOtherClass .
+        }
+    }
+    FILTER isIRI(?nClass)
+}
+"""):
+        n_class: URIRef = result[0]
+        if not str(n_class).startswith("https://ontology.unifiedcyberontology.org/uco/"):
+            continue
+        computed.add(n_class)
+    assert expected == computed