In prolog, the difference between A = B and A == B is that = tries to unify A with B, while == will only succeed if A and B are already unified. member/2 does seem to perform unification. Example session: ?- A = B. A = B. ?- A == B. false. ?- member(A,… Read More Is there a non-unifying alternative to member/2 in SWI-Prolog?