Proof
fun x H
=> op_cancel_neg_gen x H (op_neg_inv_ex x H).

(* Specifications: *)
(** Proves that negation is onto over the subset of invertable values. *)
Theorem op_neg_onto
:  forall (y : M) (H : has_inv y), exists (x : M) (H0 : has_inv x), {-} x H0 = y.