(** 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 cop3_osp__os : forall A B C D E X Y, D <> E ->
Coplanar A B C D -> Coplanar A B C E -> Coplanar D E X Y ->
OSP A B C X Y -> OS D E X Y.