docs: def _check_enclosing_characters(string, opener, closer):
      """
      Makes sure that the enclosing characters for a definition set make sense
      1) There is only one set
      2) They are in the right order (opening, then closing)
      """
      opener_count = string.count(opener)
      closer_count = string.count(closer)
      total = opener_count + closer_count
      if total > 2:
          msg = MORE_THAN_ONE_SET.format(opener, closer)
          raise ValueError(msg)
      elif total == 1:
          msg = INCOMPLETE_SET.format(opener, closer)
          raise ValueError(msg)
      elif opener_count > 1:
          msg = INCORRECT_SET_CONSTITUENT.format(opener)
          raise ValueError(msg)
      elif closer_count > 1:
          msg = INCORRECT_SET_CONSTITUENT.format(closer)
          raise ValueError(msg)