(** 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 cop_per2__col : forall A X Y Z,
Coplanar A X Y Z ->  A <> Z -> Per X Z A -> Per Y Z A -> Col X Y Z.