Lemma abelem_rV_V x : x \in E -> ErV x^-1 = - ErV x.
Proof. by move=> Ex; rewrite morphV. Qed.

(* Specifications: *)
Local Notation rV_E := rVabelem
Lemma abelem_rV_K : {in E, cancel
Lemma rVabelemK : cancel rV_E ErV. P
Proof. by rewrite morphim_invmE morphpreK. Qed.