Is true if Object can be reached from Subject following the
transitive predicate Predicate or a sub-property thereof, while
repecting the symetric(true) or inverse_of(P2) properties.
If used with either Subject or Object unbound, it first returns
the origin, followed by the reachable nodes in breadth-first
search-order. The implementation internally looks one solution
ahead and succeeds deterministically on the last solution. This
predicate never generates the same node twice and is robust
against cycles in the transitive relation.
With all arguments instantiated, it succeeds deterministically
if a path can be found from Subject to Object. Searching starts
at Subject, assuming the branching factor is normally lower. A
call with both Subject and Object unbound raises an
instantiation error. The following example generates all
subclasses of rdfs:Resource:
?- rdf_reachable(X, rdfs:subClassOf, rdfs:'Resource').
X = 'http://www.w3.org/2000/01/rdf-schema#Resource' ;
X = 'http://www.w3.org/2000/01/rdf-schema#Class' ;
X = 'http://www.w3.org/1999/02/22-rdf-syntax-ns#Property' ;
...
