Proof Abelian_Group.op_id_uniq sum_abelian_group.

(* Specifications: *)
(** Proves that for every group element, x, its left and right inverses are equal. *)
Theorem sum_inv_l_r_eq
:  forall x y : E, sum_is_inv_l x y -> forall z : E, sum_is_inv_r x z -> y = z.