(** 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_perp2__col : forall X Y Z A B,
Coplanar A B Y Z -> Perp X Y A B -> Perp X Z A B -> Col X Y Z.