<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def claim_new(self) -> Iterable[str]: """Checks for messages in the ``new`` subdirectory, moving them to ``cur`` and returning their keys. """

        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]