(* Julien Narboux *)
(* Formalization of *)
(* Tarski's axiomatic *)
Require Export Ch07_midpoint.
Unset Regular Subst Tactic.
Definition Per := fun A B C =>
exists C', is_midpoint B C C' /\ Cong A C A C'.

Lemma l8_2 : forall A B C, Per A B C -> Per C B A.