apply transitive with (plus (cons (opp u) u) (cons u v)).
apply symetrique; auto.
apply Chasles.
apply compatible.
apply oppu_u.
apply reflexive.
Qed.

(* Specifications: *)
Lemma Chasles_2 :
forall u v w t : V,
R (cons u t) (plus (cons u v) (plus (cons v w) (cons w t))).