(** 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 col_one_side : forall A B C P Q,
Col A B C -> A <> C -> OS A B P Q -> OS A C P Q.