Merge pull request #22 from cmungall/issue-21-redux

Docs for edges, see #21

src/schema/ontology_model.yaml

@@ -86,7 +86,29 @@ classes:
       - relationship
     is_a: ontology element
     description: >-
-      A relationship between two nodes. We assume owlstar or similar for existential restrictions
+      A relationship between two nodes.
+      
+      Currently the only kinds of edges supported in KGCL:
+
+        * A subClassOf B <==> Edge(subject=A, predicate=owl:subClassOf, object=B)
+        * A subClassOf P some B <==> Edge(subject=A, predicate=P, object=B)
+        * P subPropertyOf Q <==> Edge(subject=P, predicate=owl:subPropertyOf, object=Q)
+
+      These represent the most common kind of pairwise relationship between classes,
+      and classes are the dominant node type in ontologies.
+        
+      In future a wider variety of OWL axiom types will be supportedn through the use
+      of an additional edge property/slot to indicate the interpretation of the axiom,
+      following owlstar (https://github.com/cmungall/owlstar).
+
+      For example:
+       * `A subClassOf R only B <==> Edge(subject=A, predicate=P, object=B, interpretation=AllOnly)`
+       * `A Annotation(P,B) <==> Edge(subject=A, predicate=P, object=B, interpretation=annotationAssertion)`
+
+      Note that not all axioms are intended to map to edges. Axioms/triples where the object is a literal
+      would be represented as node properties. Complex OWL axioms involving nesting would have their own
+      dedicated construct, or may be represented generically. These are out of scope for the current
+      version of KGCL
     mappings:
       - owl:Axiom
       - rdf:Statement