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