proposition: Lemma mesure_mod_deuxpi : forall (x : R) (A B C D : PO), A <> B -> C <> D -> image_angle x = cons_AV (vec A B) (vec C D) -> image_angle (x + deuxpi) = cons_AV (vec A B) (vec C D).
proof: Proof. intros. rewrite add_mes_compatible. rewrite pi_plus_pi. rewrite H1. rewrite (angle_nul (A:=C) (B:=D)); auto. apply Chasles; auto. Qed.