Definition Nadd n p :=
  match n with
  | N0 => p
  | Npos p0 => (p0+p)%positive
  end.

Lemma of_uint_acc_eqn d acc : d<>Nil -> Pos.of_hex_uint_acc d acc = Pos.of_hex_uint_acc (tl d) (Nadd (hd d) (0x10*acc)).