(** 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_ts__ts : forall A B X Y Z, TS A B X Y -> Bet X Y Z -> TS A B X Z.