code: func (s *Simulation) NodeItem(id enode.ID, key interface{}) (value interface{}, ok bool) {
  	s.mu.Lock()
  	defer s.mu.Unlock()
  
  	if _, ok := s.buckets[id]; !ok {
  		return nil, false
  	}
  	return s.buckets[id].Load(key)
  }
docs: // NodeItem returns an item set in ServiceFunc function for a particular node.