Releases all of the given keys.
        :param keys: the keys to release
        :return: the names of the keys that were released

def release_all(self, keys: Sequence[str]) -> Set[str]:
        """
        Releases all of the given keys.
        :param keys: the keys to release
        :return: the names of the keys that were released
        """
        released: List[str] = []
        for key in keys:
            released.append(self.release(key))
        return set(filter(None,released))