(** * Conversions between [Z] and [N] *)
(** [Z.of_N] is a bijection between [N] and non-negative [Z], with [Z.to_N] (or [Z.abs_N]) as reciprocal. See [Z2N.id] below for the dual equation. *)

Lemma inj_sub n m : (m<=n)%N -> Z.of_N (n-m) = Z.of_N n - Z.of_N m.