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]
Checks for messages in the ``new`` subdirectory, moving them to
        ``cur`` and returning their keys.