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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _getNextId(self):
 """ Returns an unique id. By default, generates pool of UUID in increments of ``UUID_POOL_GEN_SIZE``. Could be overridden to provide custom ID generation. :return: generated unique "on the wire" message ID :rtype: str """

        if not self._uuids:
            for _ in range(self.UUID_POOL_GEN_SIZE):
                self._uuids.append(uuid.uuid4().bytes)
        return self._uuids.pop()