code: func (f *IndexFile) Open() error {
  	defer func() {
  		if err := recover(); err != nil {
  			err = fmt.Errorf("[Index file: %s] %v", f.path, err)
  			panic(err)
  		}
  	}()
  
  	// Extract identifier from path name.
  	f.id, f.level = ParseFilename(f.Path())
  
  	data, err := mmap.Map(f.Path(), 0)
  	if err != nil {
  		return err
  	}
  
  	return f.UnmarshalBinary(data)
  }
docs: // Open memory maps the data file at the file's path.