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

always (subtructLists(List, [Head|Rest], Result): -
       ( 
          delete_element(Head, List, Subtructed)
        , !
        , subtructLists(Subtructed, Rest, Result)
       ) ; (
          subtructLists(List, Rest, Result)
       )
).

always (subtructLists(List, [], List)).

always( delete_element(X, [X|Tail], Tail)).

always( delete_element(X, [Y|Tail1], [Y|Tail2]): -
        delete_element(X, Tail1, Tail2)
).