lists.pl

% Always zero elements of any element in the empty list
count(_, [], 0).
count(E, L, C) :- count(E, L, C, 0).

% This means that the count of the empty list is simply whatever
% is in the accumulator when the list is empty.
count(_, [], Acc, Acc).
count(E, [X|L], C, Acc) :- count(E, L, C, Acc).
count(X, [X|L], C, Acc) :-
    Y is Acc + 1, count(X, L, C, Y).