/* another example Prolog knowledge base */

% note: there is an issue with the brother rule below...

% last modified: 03-2-11

father(athena, zeus). 
father(ares, zeus). 
female(athena). 
male(ares). 
male(zeus). 

brother(Has_brother,Is_brother) :-
   male(Is_brother), father(Has_brother, Father), father(Is_brother, Father).