:- table a/0, b/0, a/1, b/1.

test :- a, fail.
test :- b(0), fail.
test :- b(1), fail.
test :-
	( a ->
	    ( tnot(a) -> writeln('a is undefined') ; writeln('a is true (OK)') )
	; writeln('a is false')
	),
	( b -> writeln('b is true') ; writeln('b is false (OK)') ), fail.
test :-
	( a(0) -> writeln('a(0) is true (OK)') ; writeln('a(0) is false') ),
	( X = 1, a(X) ->
		( tnot(a(1)) ->  writeln('a(1) is undefined (OK)')
		; writeln('a(1) is true') )
	; writeln('a(1) is false')
	),
	( b(0) -> writeln('b(0) is true') ; writeln('b(0) is false (OK)') ),
	( b(1) ->
		( tnot(b(1)) ->  writeln('b(1) is undefined (OK)')
		; writeln('b(1) is true') )
	; writeln('b(1) is false')
	).

a :- b.
a.

b :- tnot(a).

a(X) :- b(X).
a(0).

b(X) :- tnot(a(X)).