% join two lists
join(X, Y) -> actual_join(lists:reverse(X), Y).
actual_join(X, []) -> X;
actual_join([], X) -> X;
actual_join([X|Xs], Y) -> actual_join(Xs, [X|Y]).

% member
member(_, []) -> false;
member(X, [X|_Ys]) -> true;
member(X, [_Y|Ys]) -> member(X, Ys).