如何在prolog中获得曾曾祖母

female(ann).
female(mary).
female(jane).
female(lisa).
female(sarah).
female(emily).

male(john).
male(peter).
male(james).
male(jack).
male(david).
male(jacob).

parent(john, mary).
parent(john, peter).
parent(john, jane).
parent(mary, lisa).
parent(mary, james).
parent(peter, sarah).
parent(peter, jack).
parent(jane, emily).
parent(jane, david).
parent(lisa, jacob).

% 规则：X是Y的祖父母，如果X是Y的父母的父母
grandparent(X, Y) :- parent(X, Z), parent(Z, Y).

% 规则：X是Y的曾祖父母，如果X是Y的祖父母的父母
greatgrandparent(X, Y) :- grandparent(X, Z), parent(Z, Y).

?- greatgrandparent(X, emily).