code: func (b *BlockChain) Subscribe(callback NotificationCallback) {
  	b.notificationsLock.Lock()
  	b.notifications = append(b.notifications, callback)
  	b.notificationsLock.Unlock()
  }
docs: // Subscribe to block chain notifications. Registers a callback to be executed
  // when various events take place. See the documentation on Notification and
  // NotificationType for details on the types and contents of notifications.