parents(william, charles, dianna). parents(henry , charles, dianna). parents(charles, philip, elizabeth). parents(dianna, edward, frances). parent(X,Y) :- parents(X,Y,Z). parent(X,Y) :- parents(X,Z,Y). grandparent(X,G) :- parent(X,P), parent(P,G). ?- grandparent(william,G), write(G).