code: func (s *CombinedMapSlice) UpsertLocal(key MergeKeys, l interface{}) error {
  	item, err := s.upsert(key, l)
  	if err != nil {
  		return err
  	}
  	item.SetLocal(l)
  	return nil
  }
docs: // UpsertLocal adds l to the slice.  If there is already a value of l sharing
  // l's merge key in the slice for either the recorded or remote, set l the local value
  // Otherwise append a new item to the list with the local value.