(** * 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. *)

(** [Z.of_N] produce non-negative integers *)
Lemma is_nonneg n : 0 <= Z.of_N n.