(** Hence the number of bits of [a] is [1+log2 a] (see [Pos.size_nat] and [Pos.size]). *)
(** Testing bits after division or multiplication by a power of two *)

(** Selecting the low part of a number can be done by a modulo *)
Lemma mod_pow2_bits_high : forall a n m, n<=m ->
(a mod 2^n).[m] = false.