(** 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 ts2__ex_bet2 : forall A B C D, TS A C B D -> TS B D A C ->
exists X, Bet A X C /\ Bet B X D.