code: func extractID(s string, ln *line) (string, error) {
  	tokens := strings.Split(s, sharp)
  
  	l := len(tokens)
  
  	if l < 2 {
  		return "", nil
  	}
  
  	if l > 2 {
  		return "", fmt.Errorf("multiple IDs are specified [file: %s][line: %d]", ln.fileName(), ln.no)
  	}
  
  	return strings.Split(tokens[1], dot)[0], nil
  }
docs: // extractID extracts and returns an ID.