Proof
fun x
=> Monoid.op_inv_l_uniq op_monoid x (op_inv_r_ex x).

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