apply transitive with (plus (plus b c) a).
apply plus_sym.
apply transitive with (plus b (plus c a)).
apply symetrique; auto.
apply point_angle.plus_assoc.
apply compatible.
apply reflexive.
apply plus_sym.
Qed.

Lemma oppu_oppv : forall u v : V, R (cons (opp u) (opp v)) (cons u v).
intros u v; try assumption.