(** 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_1 :
forall A B C D X,
~ Col A B C -> Col A B X -> Out X C D ->
OS A B C D.