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:- module lte.

:- interface.

:- pred goal is semidet.

:- implementation.

:- type nat
    --->    zero
    ;       s(nat).

goal :-
    lte(X, s(s(s(s(zero))))),
    even(X).

:- pred lte(nat, nat).
:- mode lte(out, in).

lte(s(X), s(Y)) :-
    lte(X, Y).
lte(zero, _Y).

:- pred even(nat).
:- mode even(in).

even(s(s(X))) :-
    even(X).
even(zero).