Definition block_data_order_loop2 :=
   nth 1 (loops (fn_body f_sha256_block_data_order)) Sskip.Fixpoint Xarray' (b: list int) (i k: nat) : list int :=
 match k with
 | O => nil
 | S k' => W (nthi b) (Z.of_nat i - 16 + (16-(Z.of_nat k)- Z.of_nat i) mod 16) ::
                 Xarray' b i k'
 end.Definition Xarray (b: list int) (i: nat) := Xarray' b i 16.

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