(** * 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_testbit a n :
Z.testbit (Z.of_N a) (Z.of_N n) = N.testbit a n.