code: func (n *Node) Register(constructor ServiceConstructor) error {
  	n.lock.Lock()
  	defer n.lock.Unlock()
  
  	if n.server != nil {
  		return ErrNodeRunning
  	}
  	n.serviceFuncs = append(n.serviceFuncs, constructor)
  	return nil
  }
docs: // Register injects a new service into the node's stack. The service created by
  // the passed constructor must be unique in its type with regard to sibling ones.