code: def _archive_single_dir(archive):
      
  
      common_root = None
      for info in _list_archive_members(archive):
          fn = _info_name(info)
          if fn in set(['.', '/']):
              continue
          sep = None
          if '/' in fn:
              sep = '/'
          elif '\\' in fn:
              sep = '\\'
          if sep is None:
              root_dir = fn
          else:
              root_dir, _ = fn.split(sep, 1)
          if common_root is None:
              common_root = root_dir
          else:
              if common_root != root_dir:
                  return None
      return common_root
docs: Check if all members of the archive are in a single top-level directory
  
      :param archive:
          An archive from _open_archive()
  
      :return:
          None if not a single top level directory in archive, otherwise a
          unicode string of the top level directory name