docs: 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