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:- module applast_impl.

:- interface.

:- import_module list.

:- pred applast(list(T), T, T).
:- mode applast(in, in, in) is semidet.
:- mode applast(in, in, out) is semidet.
% :- mode applast(in, out, in) is semidet.

:- implementation.

applast(L, X, Last) :-
    app(L, [X], LX),
    list_last(Last, LX).

:- pred list_last(T, list(T)).
:- mode list_last(out, in) is semidet.

list_last(X, [H | T]) :-
    (
        T = [],
        X = H
    ;
        T = [_ | _],
        list_last(X, T)
    ).

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

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