(** 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_eq : forall A B X Y,
A <> X -> Col A X Y -> Col B X Y ->
~ Col A X B ->
X = Y.