code: def claim_new(self) -> Iterable[str]:
          
          new_subdir = self._paths['new']
          cur_subdir = self._paths['cur']
          for name in os.listdir(new_subdir):
              new_path = os.path.join(new_subdir, name)
              cur_path = os.path.join(cur_subdir, name)
              try:
                  os.rename(new_path, cur_path)
              except FileNotFoundError:
                  pass
              else:
                  yield name.rsplit(self.colon, 1)[0]
docs: Checks for messages in the ``new`` subdirectory, moving them to
          ``cur`` and returning their keys.