(** 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_3 : forall A B C X Y P, Coplanar A B C P -> Col X Y P ->
TSP A B C X Y <-> Bet X P Y /\ ~ Coplanar A B C X /\ ~ Coplanar A B C Y.