(** 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 outer_pasch : forall A B C P Q,
Bet A C P -> Bet B Q C -> exists X, Bet A X B /\ Bet P Q X.