(** 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 os_out_os : forall A B C D C' P , Col A B P -> OS A B C D -> Out P C C' -> OS A B C' D.