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