Definition double d :=
  match d with
  | Pos u => Pos (Unsigned.double u)
  | Neg u => Neg (Unsigned.double u)
  end.

Lemma nztail_to_hex_uint_pow16 n : Hexadecimal.nztail (Pos.to_hex_uint (Nat.iter n (Pos.mul 16) 1%positive)) = (D1 Nil, n).