(** 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 l9_8_1 : forall P Q A B C, TS P Q A C -> TS P Q B C -> OS P Q A B.