Section ORDERING.
(* B1 : If B is between A and C, then A, B, C are distinct points on a line, and also B is between C and A. *)
(* B2 : For any two distinct points A, B, there exists a points C such that B is between A and C. *)

Lemma B2 : forall A B,
A <> B ->
{C : Point | Between A B C}.