(** Lemma 16.2 in dimension 2. *)

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.