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.*)
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Lemma sign_dec : forall O E A, Col O E A -> O <> E -> A = O \/ Ps O E A \/ Ng O E A.