bird(b1).
penguin(p1).

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

% these rules with function symbols cause s(CASP) not to terminate.
% regardless of where we put these rules in the file

bird(f(X)) :- bird(X).
penguin(f(X)) :- penguin(X).


% ?- -flies(b1).

?- flies(b1).