father(b1,b2).
father(p1,p2).

bird(b1).
penguin(p1).

bird(X) :- penguin(X).
flies(X) :- bird(X), not -flies(X).
-flies(X) :- penguin(X). 

% these ancestor rules along with the father facts
% above make s(CASP) not terminate

anc(X,Y):- father(X,Y).
anc(X,Y) :- anc(X,Z), anc(Z,Y).

penguin(X):- anc(Y,X), penguin(Y).
%bird(X):- anc(Y,X), bird(Y).

% ?- -flies(p1).

?- flies(b1).