(** 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 ts_ts_os : forall A B C D, TS A B C D -> TS C D A B -> OS A C B D.