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:- module doubleapp_impl.

:- interface.

:- import_module list.

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

:- implementation.

double_app(X, Y, Z, Res) :-
    app(X, Y, Int),
    app(Int, Z, Res).

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

app([], L, L).
app([H | X], Y, [H | Z]) :-
    app(X, Y, Z).