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 bet_lt21_le31 : forall O E E' A B C,
Bet A B C -> LtP O E E' B A -> LeP O E E' C A.