docs: def _can_bag(test_dir):
      """returns (unwriteable files/folders)
      """
      unwriteable = []
      for inode in os.listdir(test_dir):
          if not os.access(os.path.join(test_dir, inode), os.W_OK):
              unwriteable.append(os.path.join(os.path.abspath(test_dir), inode))
      return tuple(unwriteable)