code: func (s *Scratch) MapSetX(k, mk string, v interface{}) (string, error) {
  	s.init()
  
  	s.Lock()
  	defer s.Unlock()
  	return s.mapSet(k, mk, v, false)
  }
docs: // MapSetX behaves the same as MapSet, except it will not overwrite the map
  // key if it already exists.