<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def extend(self, elts):
 """Adds elts to the tasks. Args: elts (Sequence):
 a iterable of elements that can be appended to the task's bundle_field. Returns: Event: an event that can be used to wait on the response. """

        # Use a copy, not a reference, as it is later necessary to mutate
        # the proto field from which elts are drawn in order to construct
        # the bundled request.
        elts = elts[:]
        self._in_deque.append(elts)
        event = self._event_for(elts)
        self._event_deque.append(event)
        return event