% max(L, M) computes as M the maximum of a non-empty list of numbers L.

max([X | L], M) :- max_helper(L, X, M).

max_helper([], M, M).

max_helper([X | L], M, N) :-
    X > M,
    max_helper(L, X, N).

max_helper([X | L], M, N) :-
    X =< M,
    max_helper(L, M, N).

test :- max([3, 7, 2, 9, 1, -5], M), write(M), nl.