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:- module append.

:- interface.

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

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

:- implementation.

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