Proof
eq_refl ({-} 1)
|| {-} 1 = a @a by prod_x_n1_neg 1
|| {-} 1 = a @a by <- prod_id_l {-1}.

(* Specifications: *)
(** Proves that the additive negation of -1 equals 1. *)
Theorem neg_n1
:  {-} {-1} = 1.