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:- module permutation.

:- interface.

:- type list(T)
    --->    []
    ;       [T | list(T)].

:- pred perm(list(T)::in, list(T)::out) is nondet.

:- implementation.

perm(Xs, [X | Ys]) :-
    app(X1s, [X | X2s], Xs),
    app(X1s, X2s, Zs),
    perm(Zs, Ys).
perm([], []).

:- pred app(list(T), list(T), list(T)).
:- mode app(out, out, in).
:- mode app(in, in, out).

app([X | Xs], Ys, [X | Zs]) :-
    app(Xs, Ys, Zs).
app([], Ys, Ys).