equivalentProperty is unnecessary

[sesuncedu]

Since (p≡q) ↔ (p ≤ q∧q ≤ p), and equivalentProperty assertions must be replicated across vocabularies anyway, requiring separate subPropertyOf assertions in the same vocabulary specification is not a great imposition, and simplifies the specification.

[gkellogg]

Point taken, but this maintained some form of compatibility with previous expansion, and RDFa vocabulary entailment. The complexity goes in the registry definition, which seems appropriate. We can leave this open to consider for a future update.