Add ancestor relation

tp/1/parent.lp

@@ -5,3 +5,6 @@ parent(lucie,robert). parent(robert,mattis). parent(robert,anna).
 child(C, P) :- parent(P, C).
 grandparent(G, C) :- parent(G, P), parent(P, C).
 sibling(S1, S2) :- parent(P, S1), parent(P, S2), S1 > S2.
+
+ancester(X, Y) :- parent(X, Y).
+ancester(X, Z) :- ancester(X, Y), ancester(Y, Z).