(** 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 osp_transitivity : forall A B C P Q R, OSP A B C P Q -> OSP A B C Q R -> OSP A B C P R.