Entity Relationships: Reified and Unreified

As explained in the W3C note about defining N-ary Relations, one commonly encounters a limitation in the OWL2-DL vocabulary for modeling relationships that, according to some domain, ought to be n-ary instead of binary like OWL2 Object Properties or OWL2 Data Properties. There are several strategies for encoding an n-ary relation; the W3C note describes a pattern based on Introducing a new class for a relation.

In OML, all entity relationships are binary (n=2). On surface, it seems that reification would be unecessary since OWL2 Object Properties and OWL2 Data Properties are binary. However, [OWL2] properties are binary and unreified; that is, neither an OWL2 Object Property nor an OWL2 Data Property can be used as the domain or range of another OWL2 Object Property or OWL2 Data Property as this would violate the Typing Constraints of OWL 2 DL unless OWL2 Metamodeling techniques are used; which effectively means that an IRI would be used for declaring, for example, an OWL2 Object Property and an OWL2 Class, thereby allowing the same IRI to be used to refer to the OWL2 Class -- e.g., for classification -- or to the OWL2 Object Property -- e.g. for specifying or restricting its domain/range.

OML ReifiedRelationship is based on a variant of the N-ary Relations Use-Case 1 encoded in OWL2-DL with SWRL rules without OWL2 Metamodeling in full compliance with the Global Restrictions on Axioms in OWL2 DL including the restrictions on OWL2 Property Hierarchy and Simple Object Property Expressions. As described in the summary, this pattern presents a practically useful benefit because it enforces a complete separation at the instance level between OML ReifiedRelationship(s) whose instances must be authored by someone because they require specifying an OWL2 NamedIndividual and OML UnreifiedRelationship(s) whose instances can be inferred thanks to automated [OWL2 DL Reasoning] with SWRL rules (e.g., with the Pellet reasoner).

TODO ... diagram about theOML ReifiedRelationship pattern, explanation, examples ...