Proof Monoid.op_inv_l_uniq prod_monoid.

(* Specifications: *)
(** Proves that an element's right inverse is unique. *)
Theorem prod_inv_r_uniq
:  forall x : E, prod_has_inv_l x -> forall y z : E, prod_is_inv_r x y -> prod_is_inv_r x z -> z = y.