% A sample database.  
   % marks comments
   % in file family.P

   % FACTS
      male(bob).             % bob is male
      parent(jill, bill).    % jill is parent of bill
      parent(bob, bill).    
   
      
   % RULES
    father(TheDad, TheChild) :- parent(TheDad, TheChild),
                male(TheDad). 

      % TheDad is the father of TheChild 
      %  if TheDad is the parent of TheChild and TheDad is male

      % Use meaningful variable names:
      %        father(X, Y) :- parent(X, Y), male(X). 
      %       X is the father of Y if X is the parent of Y 
      %                           and X is male
      
      %    ":-" read as "if"
      %    ","  read as "and"