(** 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 l9_31 :
forall A X Y Z,
OS A X Y Z ->
OS A Z Y X ->
TS A Y X Z.