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

%------------------------------
:- table a/0, b/0, f/0.

a :- f.
a.

b :- tnot(f).

f :- a, fail.