gp(X,Z) :- parent(X,Y) & parent(Y,Z)

anc(X,Z) :- parent(X,Z)
anc(X,Z) :- parent(X,Y)& anc(Y,Z)

person(X) :- parent(X,Y)
person(Y) :- parent(X,Y)

isparent(X) :- parent(X,Y)
childless(X) :- person(X) & ~isparent(X)