docs: def add_accent_at(string, accent, index):
      """
      Add mark to the index-th character of the given string.  Return
      the new string after applying change.
      (unused)
      """
      if index == -1:
          return string
      # Python can handle the case which index is out of range of given string
      return string[:index] + \
          accent.accent.add_accent_char(string[index], accent) + \
          string[index+1:]