code: func OpenReadSegment(fn string) (*Segment, error) {
  	k, err := strconv.Atoi(filepath.Base(fn))
  	if err != nil {
  		return nil, errors.New("not a valid filename")
  	}
  	f, err := os.Open(fn)
  	if err != nil {
  		return nil, err
  	}
  	return &Segment{File: f, i: k, dir: filepath.Dir(fn)}, nil
  }
docs: // OpenReadSegment opens the segment with the given filename.