(** This lemma used to be an axiom in previous versions of Tarski's axiom system. It is a been shown to a theorem by Gupta in his Phd 1965. *)
(** This corresponds to l9_6 in Tarski's book. *)

Lemma bet_cop__cop : forall A B C D E, Coplanar A B C E -> Bet C D E -> Coplanar A B C D.