Corollary Ztestbit_two_p_m1:
  forall n i, 0 <= n -> 0 <= i ->
  Z.testbit (two_p n - 1) i = if zlt i n then true else false.

Theorem sign_zero_ext_widen: forall x n n', 0 <= n < n' -> sign_ext n' (zero_ext n x) = zero_ext n x.