code: func (s *Scanner) err(msg string) {
  	s.ErrorCount++
  	pos := s.recentPosition()
  
  	if s.Error != nil {
  		s.Error(pos, msg)
  		return
  	}
  
  	fmt.Fprintf(os.Stderr, "%s: %s\n", pos, msg)
  }
docs: // err prints the error of any scanning to s.Error function. If the function is
  // not defined, by default it prints them to os.Stderr