code: def format_label(sl, fmt=None):
      
      if isinstance(sl, str):
          # Already is a string.
          return sl
  
      if fmt:
          return fmt.format(*sl)
  
      return ' '.join(str(s) for s in sl)
docs: Combine a list of strings to a single str, joined by sep.
      Passes through single strings.
      :param sl:
      :return: