proposition: Lemma mes_oppx : forall (A B C D : PO) (x : R), A <> B -> C <> D -> image_angle x = cons_AV (vec A B) (vec C D) -> image_angle (- x) = cons_AV (vec C D) (vec A B).
proof: Proof. intros. rewrite <- def_opp; auto. rewrite <- H1; rewrite <- mes_opp; auto with geo. Qed.