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 15.5. *)
(** Known as Thales theorem or intercept theorem. *)
Lemma length_out : forall O E E' A B C D AB CD,
A <> B -> C <> D -> Length O E E' A B AB -> Length O E E' C D CD ->
Out O AB CD.