(** 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 coplanar_trans_1 : forall P Q R A B,
~ Col P Q R -> Coplanar P Q R A -> Coplanar P Q R B -> Coplanar Q R A B.