% perm(Xs, Ys) :- Ys is a permutation of the list Xs.
perm([], []).
perm(Xs, [X | Ys]) :-
	app(X1s, [X | X2s], Xs),
	app(X1s, X2s, Zs),
	perm(Zs, Ys).

% augmented by the APPEND program
app([], Ys, Ys).
app([X | Xs], Ys, [X | Zs]) :- app(Xs, Ys, Zs).