1% Given 3 birds, which can fly?
    2
    3#pred penguin(X) :: '@(X) is a penguin'.
    4penguin(sam). % sam is a penguin
    5#pred wounded_bird(X) :: '@(X) is a wounded bird'.
    6wounded_bird(john). % john is wounded
    7
    8#pred bird(X) :: '@(X) is a bird'.
    9bird(tweety). % tweety is just a bird
   10% penguines and wounded birds are still birds
   11bird(X) :- penguin(X).
   12bird(X) :- wounded_bird(X).
   13
   14#pred ab(X) :: '@(X:bird) is abnormal'.
   15% penguins and wounded birds are abnormal
   16ab(X) :- penguin(X).
   17ab(X) :- wounded_bird(X).
   18
   19#pred flies(X) :: '@(X) can fly'.
   20% birds can fly if they are not abnormal
   21flies(X) :- bird(X), not ab(X).
   22
   23#pred -flies(X) :: '@(X) can not fly'.
   24% explicit closed world assumptions
   25-flies(X) :- ab(X).
   26-flies(X) :- -bird(X).
   27
   28-wounded_bird(X) :- not wounded_bird(X).
   29
   30-bird(X) :- not bird(X).
   31
   32-penguin(X) :- not penguin(X).
   33
   34-ab(X) :- not ab(X).
   35
   36
   37?- flies(X).