There is a subset called "pure Prolog", which is restricted to Horn clauses. But regular Prolog (with extralogical features) is not functional, and definitely n

There is a subset called "pure Prolog", which is restricted to Horn clauses. But regular Prolog (with extralogical features) is not functional, and definitely not pure. For instance, the proof_me predicate in the following program will be true, only once: :- dynamic bla/0. proof_me :- \+ bla, assert(bla).