Section T15.
Context `{T2D:Tarski_2D}.
Context `{TE:@Tarski_euclidean Tn TnEQD}.
(** Lemma 15.2 *)
(** Cong corresponds to length equality.*)
(** Le corresponds to length inequality.*)

Lemma is_length_exists : forall O E E' X Y,
~ Col O E E' -> exists XY, Is_length O E E' X Y XY.