(** Lemma 16.2 in dimension 2. *)

(** As we are in dimension 2, we skip 16.8 which is only needed to prove 16.11 in dimension n. *)
Lemma l16_9_1 : forall O E E' X Y XY XMY,
Col O E X -> Col O E Y -> Is_length O E E' X Y XY ->
LeP O E E' Y X -> Diff O E E' X Y XMY ->
XY = XMY.