Section Incr.
(** Variant of [incr] via [recrbis] *)
Let Incr (b : digits) (si rec : int31) :=
match b with
| D0 => sneakl D1 si
| D1 => sneakl D0 rec
end.
Definition incrbis_aux n x := recrbis_aux n _ In Incr x.
(** Recursive equations satisfied by [incr] *)

Lemma incr_eqn1 :
forall x, firstr x = D0 -> incr x = twice_plus_one (shiftr x).