male(albert).
female(alice).
male(edward).
female(victoria).

parent(albert,edward).
parent(victoria,edward).
parent(albert,alice).
parent(victoria,alice).
parent(alice, bob).
parent(bob, sheila).  

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