Bird(X) <- Chicken(X).
Bird(X) <- Penguin(X).
~Flies(X) <- Penguin(X).
Chicken(tina).
Penguin(tweety).
Scared(tina).

Flies(X) -< Bird(X).
~Flies(X) -< Chicken(X).
Flies(X) -< Chicken(X), Scared(X).
Nests_in_trees(X) -< Flies(X).
