Section T16.
Context `{T2D:Tarski_2D}.
Context `{TE:@Tarski_euclidean Tn TnEQD}.
(** Lemma 16.2 in dimension 2. *)

Lemma Cs_not_Col : forall O E S U1 U2, Cs O E S U1 U2 -> ~ Col U1 S U2.