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 length_Ar2 : forall O E E' A B AB,
Length O E E' A B AB -> (Col O E AB /\ ~Col O E E') \/ AB = O.