code: func (b *BlockChain) sendNotification(typ NotificationType, data interface{}) {
  	// Generate and send the notification.
  	n := Notification{Type: typ, Data: data}
  	b.notificationsLock.RLock()
  	for _, callback := range b.notifications {
  		callback(&n)
  	}
  	b.notificationsLock.RUnlock()
  }
docs: // sendNotification sends a notification with the passed type and data if the
  // caller requested notifications by providing a callback function in the call
  // to New.