gistfile1.txt

contains(X, [X | _]).
contains(X, [_ | T]) :- contains(X, T).

equals([], []).
equals([_|T1], [_|T2]) :- equals(T1, T2).

len([], 0).
len([_|T], L) :- len(T, L1), succ(L1, L).

empty([]).

first([H|_], H).

last([_,L], L).
last([_|T], L) :- last(T, L).

unshift(X, L, L1) :- L1 = [X|L].

push(X, [], [X]).
push(X, [H|T], [H|Y]) :- push(X, T, Y).

pop([X], []).
pop([H|T], [H|T1]) :- pop(T, T1). 

at([X], 0, X).
at([H|T], Y, Z) :- at(T, Y1, Z), succ(Y, Y1).
    

zip([], [], []).
zip([H|T], [H1|T1], X) :- X = [H,H1|Y], zip(T, T1, Y).