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

mem2(H, [H|_]) :- !.
mem2(X, [_|T]) :- mem2(X, T).

remove(H, [H|T], T).
remove(X, [H|T], [H|Z]) :- remove(X, T, Z).

remove2(H, [H|T], T) :- !.
remove2(X, [H|T], [H|Z]) :- remove2(X, T, Z).

removeAll(_, [ ], [ ]).
removeAll(H, [H|T], Z) :- removeAll(H, T, Z).
removeAll(X, [H|T], [H|Z]) :- not(X=H), removeAll(X, T, Z).

permute([ ], [ ]).
permute(L, [H|T]) :- member(H, L), remove(H, L, Z), permute(Z, T).

ascend([ ]).
ascend([_]).
ascend([A,B|T]) :- A =< B, ascend([B|T]).

perm_sort(L, R) :- permute(L, R), ascend(R).

tree1([h, [d, [b, [a], [c]], [f, [e], [g]]], [n, [k, [j], [m]], [q, [p], [r]]]]).

postorder([X], [X]).
postorder([X, L, R], Z) :-  postorder(L, A), postorder(R, B),
                            append(B, [X], C), append(A, C, Z).


not(X) :- \+ X.