code: func (sc *SubscriptionScope) Close() {
  	sc.mu.Lock()
  	defer sc.mu.Unlock()
  	if sc.closed {
  		return
  	}
  	sc.closed = true
  	for s := range sc.subs {
  		s.s.Unsubscribe()
  	}
  	sc.subs = nil
  }
docs: // Close calls Unsubscribe on all tracked subscriptions and prevents further additions to
  // the tracked set. Calls to Track after Close return nil.