parent(kim, holly).
parent(margaret, kim).
parent(margaret, kent).
parent(esther, margaret).
parent(herbert, margaret).
parent(herbert, jean).

greatgrandparent(GGP, CGC) :- parent(GGP, GP), parent(GP, P), parent(P, GGC).

ancestor(X, Y) :- parent(X, Y).
ancestor(X, Y) :- parent(Z, Y), ancestor(X, Z).