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_not_col_null : forall O E E' A B AB,
Col O E E' -> Length O E E' A B AB -> AB=O.