(** 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__one_or_two_sides :
forall A B C D, Coplanar A B C D ->
~ Col C A B ->
~ Col D A B ->
TS A B C D \/ OS A B C D.