code: func NewSubscriptionManager(ntfnSource NotificationSource) *SubscriptionManager {
  	return &SubscriptionManager{
  		subscribers:         make(map[uint64]*newSubscription),
  		newSubscriptions:    make(chan *newSubscription),
  		cancelSubscriptions: make(chan *cancelSubscription),
  		ntfnSource:          ntfnSource,
  		quit:                make(chan struct{}),
  	}
  }
docs: // NewSubscriptionManager creates a subscription manager backed by a
  // NotificationSource.