# Test program
male(Godwin).
male(Tostig).
male(Harold II).
male(Skule).
male(Harold).

female(Gytha).
female(Judith).
female(Gudrun).
female(Ealdgyth).
female(Edith).

parent(Godwin, Harold II).   % King of England
parent(Godwin, Tostig).
parent(Godwin, Edith).
parent(Gytha, Harold II).
parent(Gytha, Tostig).
parent(Gytha, Edith).
parent(Tostig, Skule).
parent(Judith, Skule).
parent(Harold II, Harold).
parent(Ealdgyth, Harold).

father($X, $Y) :- parent($X, $Y), male($X).
mother($X, $Y) :- parent($X, $Y), female($X).
grandmother($X, $Y) :- parent($X, $Z), parent($Z, $Y), female($X).
grandfather($X, $Y) :- parent($X, $Z), parent($Z, $Y), male($X).