docs: def add_mark_char(char, mark):
      """
      Add mark to a single char.
      """
      if char == "":
          return ""
      case = char.isupper()
      ac = accent.get_accent_char(char)
      char = accent.add_accent_char(char.lower(), Accent.NONE)
      new_char = char
      if mark == Mark.HAT:
          if char in FAMILY_A:
              new_char = "â"
          elif char in FAMILY_O:
              new_char = "ô"
          elif char in FAMILY_E:
              new_char = "ê"
      elif mark == Mark.HORN:
          if char in FAMILY_O:
              new_char = "ơ"
          elif char in FAMILY_U:
              new_char = "ư"
      elif mark == Mark.BREVE:
          if char in FAMILY_A:
              new_char = "ă"
      elif mark == Mark.BAR:
          if char in FAMILY_D:
              new_char = "đ"
      elif mark == Mark.NONE:
          if char in FAMILY_A:
              new_char = "a"
          elif char in FAMILY_E:
              new_char = "e"
          elif char in FAMILY_O:
              new_char = "o"
          elif char in FAMILY_U:
              new_char = "u"
          elif char in FAMILY_D:
              new_char = "d"
  
      new_char = accent.add_accent_char(new_char, ac)
      return utils.change_case(new_char, case)