% P1 is the father of P2 if P1 is the parent of P2 and P1 is male.

father(P1,P2) :- 
  parent(P1,P2),
  male(P1).

% P1 is a grandparent of P2 if P1 is the parent of some X and X is a
% parent of P2.

grandparent(P1,P2) :-
  parent(P1,X),
  parent(X,P2).