code: func (sc *SubscriptionScope) Track(s Subscription) Subscription {
  	sc.mu.Lock()
  	defer sc.mu.Unlock()
  	if sc.closed {
  		return nil
  	}
  	if sc.subs == nil {
  		sc.subs = make(map[*scopeSub]struct{})
  	}
  	ss := &scopeSub{sc, s}
  	sc.subs[ss] = struct{}{}
  	return ss
  }
docs: // Track starts tracking a subscription. If the scope is closed, Track returns nil. The
  // returned subscription is a wrapper. Unsubscribing the wrapper removes it from the
  // scope.