rule_test.m

:- module rule_test.
:- interface.

:- type a ---> a1 ; a2.
:- type b ---> b1 ; b2.

% Verify that if a is a1, b is b2.
:- pred check(a::in, b::in) is semidet.

:- implementation.

% No rule for a1 and b1, since we said that if a1 then b must be b2.
check(a1, b2).
check(a2, b1).
check(a2, b2).

% Using an underscore means we don't care about the value.
% So alternatively, we could have written:
% check(a1, b2).
% check(a2, _).