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:- module pl4_01.

:- interface.

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

:- pred append3(list(T), list(T), list(T), list(T)).
:- mode append3(in, in, in, out) is det.
:- mode append3(out, out, out, in) is multi.

:- implementation.

append3(A, B, C, D) :-
    append(A, B, E),
    append(E, C, D).

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

append([], L, L).
append([H | L1], L2, [H | L3]) :-
    append(L1, L2, L3).