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 opp_2_le_le : forall O E E' A MA B MB,
Opp O E E' A MA -> Opp O E E' B MB -> LeP O E E' A B -> LeP O E E' MB MA.