(** 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 col2_os__os : forall A B C D X Y, C <> D -> Col A B C ->
Col A B D -> OS A B X Y -> OS C D X Y.