\begin{verbatim}

%a \vee b  :-  q.
% q or r :- .

 p :- f, not p.

 q :- not r.
 r :- not q.

 f :- q, a', b'.

 a :- not a', q.
 a' :- not a, q.

 b :- not b', q.
 b' :- not b, q.

% c :- not c', q.
% c' :- not c, q.

 hide a'.
 hide b'.
 hide c'.
 hide p.

\end{verbatim}