<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def reset_group_subscription(self):
 """Reset the group's subscription to only contain topics subscribed by this consumer."""

        if self._user_assignment:
            raise IllegalStateError(self._SUBSCRIPTION_EXCEPTION_MESSAGE)
        assert self.subscription is not None, 'Subscription required'
        self._group_subscription.intersection_update(self.subscription)