Prolog, X before Y in a List

Question

I am having trouble understanding prolog, I have to find if X is before Y in a list.

so I have a base case with an empty list

before(X, Y, [ ]).
<         


        

Answer 1

You can use append/3 to locate X and the remaining list after it, then locate Y.

before(X, Y, L):-
  append(_, [X|Tail], L),
  append(_, [Y|_], Tail).