code: func (mem *memNamedMapping) ListNames(field string, value string) []string {
  	mem.access.RLock()
  	defer mem.access.RUnlock()
  
  	ix, found := mem.indexes[field]
  	if !found {
  		return nil
  	}
  	set, found := ix[value]
  
  	if !found {
  		return nil
  	}
  
  	return set.content()
  }
docs: // ListNames looks up the items by secondary indexes. It returns all
  // names matching the selection.