(** 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_18 : forall X Y A B P,
Col X Y P -> Col A B P -> (TS X Y A B <-> (Bet A P B /\ ~Col X Y A /\ ~Col X Y B)).