1% Given 3 birds, which can fly?
    2
    3penguin(sam). % sam is a penguin
    4wounded_bird(john). % john is wounded
    5bird(tweety). % tweety is just a bird
    6
    7% penguins and wounded birds are still birds
    8bird(X) :- penguin(X).
    9bird(X) :- wounded_bird(X).
   10
   11% penguins and wounded birds are abnormal
   12ab(X) :- penguin(X).
   13ab(X) :- wounded_bird(X).
   14
   15% birds can fly if they are not abnormal
   16flies(X) :- bird(X), not