<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def flush(self, timeout=None, callback=None):
 """Alias for self.client.flush"""

        client, scope = self._stack[-1]
        if client is not None:
            return client.flush(timeout=timeout, callback=callback)