def _process_result(self, result, session, server, sock_info, slave_ok):
        """Callback that caches the startAtOperationTime from a changeStream
        aggregate command response containing an empty batch of change
        documents.

        This is implemented as a callback because we need access to the wire
        version in order to determine whether to cache this value.
        """
        if not result['cursor']['firstBatch']:
            if (self._start_at_operation_time is None and
                    self.resume_token is None and
                    sock_info.max_wire_version >= 7):
                self._start_at_operation_time = result["operationTime"]
Callback that caches the startAtOperationTime from a changeStream
        aggregate command response containing an empty batch of change
        documents.

        This is implemented as a callback because we need access to the wire
        version in order to determine whether to cache this value.
        
stackoverflow/venv/lib/python3.6/site-packages/pymongo/change_stream.py