mother(mary, dorothy).
mother(dorothy, geoff).
mother(dorothy, graham).
mother(dorothy, barbara).
mother(barbara, chris).
mother(barbara, lucy).

father(norman, geoff).
father(norman, graham).
father(norman, barbara).
father(leon, chris).
father(leon, lucy).
father(george, dorothy).

ancestor(Ancestor, Person) :- ancestoriter(Ancestor, Person, 0).

ancestoriter(Ancestor, Ancestor, N) :-
	N > 0.

ancestoriter(Ancestor, Person, N) :-
	father(Father, Person),
	M is N + 1,
	ancestoriter(Ancestor, Father, M).

ancestoriter(Ancestor, Person, N) :-
	mother(Mother, Person),
	M is N + 1,
	ancestoriter(Ancestor, Mother, M).

[debug] 4 ?- ancestor(X,lucy).
 T Call: (8) ancestoriter(_G463, lucy, 0)
 T Redo: (8) ancestoriter(_G463, lucy, 0)
 T Call: (9) father(_L192, lucy)
 T Exit: (9) father(leon, lucy)
 T Call: (9) ancestoriter(_G463, leon, 1)
 T Exit: (9) ancestoriter(leon, leon, 1)
 T Exit: (8) ancestoriter(leon, lucy, 0)
X = leon ;
 T Redo: (9) ancestoriter(_G463, leon, 1)
 T Call: (10) father(_L229, leon)
 T Fail: (10) father(_L229, leon)
 T Redo: (9) ancestoriter(_G463, leon, 1)
 T Call: (10) mother(_L229, leon)
 T Fail: (10) mother(_L229, leon)
 T Redo: (9) father(_L192, lucy)
 T Redo: (8) ancestoriter(_G463, lucy, 0)
 T Call: (9) mother(_L192, lucy)
 T Exit: (9) mother(barbara, lucy)
 T Call: (9) ancestoriter(_G463, barbara, 1)
 T Exit: (9) ancestoriter(barbara, barbara, 1)
 T Exit: (8) ancestoriter(barbara, lucy, 0)
X = barbara ;
 T Redo: (9) ancestoriter(_G463, barbara, 1)
 T Call: (10) father(_L205, barbara)
 T Exit: (10) father(norman, barbara)
 T Call: (10) ancestoriter(_G463, norman, 2)
 T Exit: (10) ancestoriter(norman, norman, 2)
 T Exit: (9) ancestoriter(norman, barbara, 1)
 T Exit: (8) ancestoriter(norman, lucy, 0)
X = norman ;
 T Redo: (10) ancestoriter(_G463, norman, 2)
 T Call: (11) father(_L242, norman)
 T Fail: (11) father(_L242, norman)
 T Redo: (10) ancestoriter(_G463, norman, 2)
 T Call: (11) mother(_L242, norman)
 T Fail: (11) mother(_L242, norman)
 T Redo: (10) father(_L205, barbara)
 T Fail: (10) father(_L205, barbara)
 T Redo: (9) ancestoriter(_G463, barbara, 1)
 T Call: (10) mother(_L205, barbara)
 T Exit: (10) mother(dorothy, barbara)
 T Call: (10) ancestoriter(_G463, dorothy, 2)
 T Exit: (10) ancestoriter(dorothy, dorothy, 2)
 T Exit: (9) ancestoriter(dorothy, barbara, 1)
 T Exit: (8) ancestoriter(dorothy, lucy, 0)
X = dorothy ;
 T Redo: (10) ancestoriter(_G449, dorothy, 2)
 T Call: (11) father(_L235, dorothy)
 T Exit: (11) father(george, dorothy)
 T Call: (11) ancestoriter(_G449, george, 3)
 T Exit: (11) ancestoriter(george, george, 3)
 T Exit: (10) ancestoriter(george, dorothy, 2)
 T Exit: (9) ancestoriter(george, barbara, 1)
 T Exit: (8) ancestoriter(george, lucy, 0)
X = george ;
 T Redo: (11) ancestoriter(_G449, george, 3)
 T Call: (12) father(_L255, george)
 T Fail: (12) father(_L255, george)
 T Redo: (11) ancestoriter(_G449, george, 3)
 T Call: (12) mother(_L255, george)
 T Fail: (12) mother(_L255, george)
 T Redo: (10) ancestoriter(_G449, dorothy, 2)
 T Call: (11) mother(_L235, dorothy)
 T Exit: (11) mother(mary, dorothy)
 T Call: (11) ancestoriter(_G449, mary, 3)
 T Exit: (11) ancestoriter(mary, mary, 3)
 T Exit: (10) ancestoriter(mary, dorothy, 2)
 T Exit: (9) ancestoriter(mary, barbara, 1)
 T Exit: (8) ancestoriter(mary, lucy, 0)
X = mary ;
 T Redo: (11) ancestoriter(_G449, mary, 3)
 T Call: (12) father(_L265, mary)
 T Fail: (12) father(_L265, mary)
 T Redo: (11) ancestoriter(_G449, mary, 3)
 T Call: (12) mother(_L265, mary)
 T Fail: (12) mother(_L265, mary)
 T Redo: (11) mother(_L235, dorothy)
 T Fail: (11) mother(_L235, dorothy)
 T Redo: (10) mother(_L205, barbara)
 T Fail: (10) mother(_L205, barbara)
false.