Lemma l16_9_2 : forall O E E' X Y XY XMY XY2 XMY2, Col O E X -> Col O E Y -> Is_length O E E' X Y XY -> Diff O E E' X Y XMY -> Prod O E E' XY XY XY2 -> Prod O E E' XMY XMY XMY2 -> XY2 = XMY2.