%
% From Przymusinska and Przymusinski (weakly stratified paper).
% Example of a program that has a well-founded (and unique stable)
% model M = {}, but does not have a weakly perfect model.
%

:- table p/0, q/0, r/0, s/0.

p :- tnot(s), q, tnot(r).
q :- r, tnot(p).
r :- p, tnot(q).
s :- tnot(p), tnot(q), tnot(r), writeln('Negations are OK'), fail.

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

test :-
	( p -> writeln('p is true') ; writeln('p is false (OK)') ),
	( q -> writeln('q is true') ; writeln('q is false (OK)') ),
	( r -> writeln('r is true') ; writeln('r is false (OK)') ),
	( s -> writeln('s is true') ; writeln('s is false (OK)') ).

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