domains
   name = symbol

predicates
   father(name, name)
   everybody
   % enumerate_fathers
   % enumerate_fathers(integer)

clauses
   father(leonard, katherine).
   father(carl, jason).
   father(carl, marilyn).
   everybody :- 
      father(X, Y),
      write(X, " is ", Y, "'s father\n"),
      % write(X, " is ", Y, "'s father\n"),
      fail.