(** 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 os_distincts : forall A B X Y, OS A B X Y ->
A <> B /\ A <> X /\ A <> Y /\ B <> X /\ B <> Y.