code: func (m *SubscriptionManager) handleCancelSubscription(msg *cancelSubscription) {
  	// First, we'll attempt to look up an existing susbcriber with the given
  	// ID.
  	sub, ok := m.subscribers[msg.id]
  	if !ok {
  		return
  	}
  
  	log.Infof("Canceling block subscription: id=%d", msg.id)
  
  	// If there is one, we'll stop their internal queue to no longer deliver
  	// notifications to them.
  	delete(m.subscribers, msg.id)
  	sub.cancel()
  }
docs: // handleCancelSubscription handles a request to cancel an existing
  // subscription.