code: func adjustErr(err error, adj int) error {
  	if err == nil {
  		return nil
  	}
  	if de, ok := err.(demangleErr); ok {
  		de.off += adj
  		return de
  	}
  	return err
  }
docs: // adjustErr adjusts the position of err, if it is a demangleErr,
  // and returns err.