(** 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 out_out_one_side :
forall A B X Y Z,
OS A B X Y ->
Out A Y Z ->
OS A B X Z.