<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def append_response(self, response):
 """Append the response to the stack of responses. :param tornado.httpclient.HTTPResponse response: The HTTP response """

        self._responses.append(response)
        if 'Warning' in response.headers:
            LOGGER.warning(
                'HTTP %s %s Warning (%s): %s (attempt %s)',
                response.request.method, response.request.url,
                response.code, response.headers['Warning'],
                len(self._responses))