(** 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 l9_19 : forall X Y A B P ,
Col X Y P -> Col A B P -> (OS X Y A B <-> (Out P A B /\ ~Col X Y A)).