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:- module arit_exp.

:- interface.

:- type expr
    --->    expr + expr
    ;       expr * expr
    ;       - expr
    ;       int(int).

:- pred e(expr).
:- mode e(in) is semidet.

:- implementation.

e(X+Y) :-
    f(X),
    e(Y).
e(X) :-
    f(X).

:- pred f(expr).
:- mode f(in) is semidet.

f(X*Y) :-
    g(X),
    f(Y).
f(X) :-
    g(X).

:- pred g(expr).
:- mode g(in) is semidet.

g(-(X)) :-
    e(X).
g(int(_X)).