code: func checkFileVersion(r io.Reader) error {
  	fv, err := util.ReadInt(r)
  	if err != nil {
  		return fmt.Errorf("unable to verify file version: %v", err)
  	}
  	if fv == 0 || fv > fileVersion {
  		return fmt.Errorf("unsupported file version: %v (supports [1..%v])", fv, fileVersion)
  	}
  	return nil
  }
docs: // check that the version of the file is understood by this interface