(** 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 l8_21_bis : forall A B C X Y, X <> Y -> ~ Col C A B -> exists P : Tpoint,
Cong A P X Y /\ Perp A B P A /\ TS A B C P.