- rdf_has(?Subject, +Predicate, ?Object, -RealPredicate) is nondet
- Same as rdf_has/3, but RealPredicate is unified to the actual
predicate that makes this relation true. RealPredicate must be
Predicate or an rdfs:subPropertyOf Predicate. If an inverse
match is found, RealPredicate is the term
inverse_of(Pred).