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

%-----------------------------------------------------------------------------
%%
%% Non left-to-right modularly-stratified program (but it is working fine...)
%%

:- table a/0, b/0, c/0, d/0.

a :- tnot(c), tnot(b).

b :- tnot(d).
b.

c :- tnot(d).

d :- tnot(b).

