Proof
conj sum_0_neg
(fun x H => eq_sym (sum_neg_0 x H)).
(** Accepts one element, x, and asserts that x is the left identity element. *)
Definition prod_is_id_l := Monoid.is_id_l E prod.
(** Accepts one element, x, and asserts that x is the right identity element. *)
Definition prod_is_id_r := Monoid.is_id_r E prod.
(** Accepts one element, x, and asserts that x is the identity element. *)
Definition prod_is_id := Monoid.is_id E prod.
(** Represents the monoid formed by op over E. *)
Definition prod_monoid := Monoid.monoid E 1 {#} prod_is_assoc prod_id_l prod_id_r.

(** Proves that 1 is the identity element. *)
Theorem prod_id
:  prod_is_id 1.