Hint Resolve Znth_land_is_int.

Lemma extract_from_b:
forall b i n,
length b = 16%nat ->
16 <= i < 64 ->
(0 <= n < 16)%Z ->
nthi (Xarray b i) ((Z.of_nat i + n) mod 16) = W (nthi b) (Z.of_nat i - 16 + n).