Section Order.
Context `{T2D:Tarski_2D}.
Context `{TE:@Tarski_euclidean Tn TnEQD}.
(* sum_preserves leP : a <= x /\ b <= y => a + b <= x + y *)
(** Lemma 14.40 states that we have an ordered field. *)

Lemma col_pos_or_neg : forall O E X,
O <> E -> O <> X -> Col O E X -> Ps O E X \/ Ng O E X.