(* Julien Narboux *)
(* Formalization of *)
(* Tarski's axiomatic *)
Require Export Ch04_congruence_between_properties.
(** Outer connectivity for betweenness Axiom III of Tarski Axiom 18 of Givant *)
(** This proof is from Gupta 1965, it used to be an axiom in previous versions of Tarski's axiomatic *)

(* Specifications: *)
Lemma l5_1 : forall A B C D,
A<>B -> Bet A B C -> Bet A B D -> Bet A C D \/ Bet A D C.