       iex> ISO.country_code_to_name("TX")
       nil
   """
-  @spec country_code_to_name(String.t()) :: nil | String.t()
+  @spec country_code_to_name(country_code()) :: nil | String.t()
   def country_code_to_name(code) when is_bitstring(code) do
     countries()[code]
   end

      iex> ISO.find_subdivision_code("SG", "SG-Invalid")
      {:error, "Invalid subdivision 'SG-Invalid' for country: SG (SG)"}
  """
  @spec find_subdivision_code(String.t(), String.t()) :: {:ok, String.t()} | {:error, String.t()}
  def find_subdivision_code(country, subdivision) do
    country_code =
      case @iso do

      iex> ISO.subdivision_code("MX", "Not a subdivision.")
      nil
  """
  @spec subdivision_code(country_code(), String.t()) :: nil | String.t()
  def subdivision_code(country, subdivision)
      when is_bitstring(subdivision) and is_bitstring(country) do
    divisions = @iso[country]["subdivisions"]
