Note that there's another flaw: `transitive` as defined only checks if there's one instance of transitivity that works. If you wanted to check (rather than assert) transitivity, you'd need something more like

    transitive(Functor) :-
        not(
            call(Functor,A,B),
            call(Functor,B,C),
            not(call(Functor,A,C))
        ).

(It's been a while since I've Prologged, so I may be forgetting something subtle about why you'd need to use the dribble `=..` to build something callable; but that's not the point, of course!)