code: func VersionLabelForChecksum(checksum string) string {
  	// Checksums are in the form: {versionLabel}+{hashID}:{hex}
  	sepIndex := strings.Index(checksum, "+")
  	if sepIndex < 0 {
  		return ""
  	}
  	return checksum[:sepIndex]
  }
docs: // VersionLabelForChecksum returns the label for the given tarsum
  // checksum, i.e., everything before the first `+` character in
  // the string or an empty string if no label separator is found.