Proof
fun x y z
=> Monoid.op_cancel_r op_monoid x y z (op_inv_r_ex z).

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