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 15.4. *)
(** Triangular equality. *)
Lemma length_uniqueness : forall O E E' A B AB AB',
Length O E E' A B AB -> Length O E E' A B AB' -> AB = AB'.