(** 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 one_side_chara : forall P Q A B,
OS P Q A B -> (forall X, Col X P Q -> ~ Bet A X B).