% append([1,2,3], [4,5,6], [1,2,3,4,5,6])
% is_list(X).

append([], Y, Y) :- is_list(Y).
append([X | Xs], Y, [X | R]) :- append(Xs, Y, R).

% rev([1,2,3], [3,2,1]).
rev([], []).
rev([X|Xs], Y) :- rev(Xs, Revs), append(Revs, [X], Y).

% elem([a,b,c], 1, b).
elem([X|_], 0, X).
% elem([_|Xs], N, Y) :- K is N - 1, elem(Xs, K, Y).
% elem([_|Xs], N, Y) :- elem(Xs, K, Y), N is K + 1.