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:- module dds1_2.

:- interface.

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

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

:- implementation.

permute([], []).
permute([X | Y], [U | V]) :-
    delete(U, [X | Y], W),
    permute(W, V).

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

delete(X, [X | Y], Y).
delete(U, [X | Y], [X | Z]) :-
    delete(U, Y, Z).