(** [Z.of_N] produce non-negative integers *)

(** [Z.of_nat] is a bijection between [nat] and non-negative [Z], with [Z.to_nat] (or [Z.abs_nat]) as reciprocal. See [Z2Nat.id] below for the dual equation. *)
Lemma id n : Z.to_nat (Z.of_nat n) = n.