Prolog list difference routine

Question

I am trying to implement a list difference routine in prolog. For some reason the following fails:

difference(Xs,Ys,D) :- difference(Xs,Ys,[],D).
difference(         


        

Answer 1

minus([H|T1],L2,[H|L3]):-
    not(member(H,L2)),
    minus(T1,L2,L3).
minus([H|T1],L2,L3):-
    member(H,L2),
    minus(T1,L2,L3).
minus([],_,[]). 

---

minus([1,2,3,4,3], [1,3], L).

output: L=[2,4]