code: func (n *Node) Service(service interface{}) error {
  	n.lock.RLock()
  	defer n.lock.RUnlock()
  
  	// Short circuit if the node's not running
  	if n.server == nil {
  		return ErrNodeStopped
  	}
  	// Otherwise try to find the service to return
  	element := reflect.ValueOf(service).Elem()
  	if running, ok := n.services[element.Type()]; ok {
  		element.Set(reflect.ValueOf(running))
  		return nil
  	}
  	return ErrServiceUnknown
  }
docs: // Service retrieves a currently running service registered of a specific type.