code: func HasID3v1Tag(r readerutil.SizeReaderAt) (bool, error) {
  	if r.Size() < ID3v1TagLength {
  		return false, nil
  	}
  
  	buf := make([]byte, len(id3v1Magic))
  	if _, err := r.ReadAt(buf, r.Size()-ID3v1TagLength); err != nil {
  		return false, fmt.Errorf("Failed to read ID3v1 data: %v", err)
  	}
  	if bytes.Equal(buf, id3v1Magic) {
  		return true, nil
  	}
  	return false, nil
  }
docs: // HasID3v1Tag returns true if an ID3v1 tag is present at the end of r.