test :- a, fail.
test :- b, fail.
test :-
	( a -> writeln('a is true') ; writeln('a is false') ),
	( b -> writeln('b is true') ; writeln('b is false') ).

%--------------------------------------------------------------

:- table a/0, b/0.

a :- tnot(b).
a.

b :- tnot(a).