code: func Err(exit int, err error, format string, a ...interface{}) {
  	format = fmt.Sprintf("[%s] %s", progname, format)
  	format += ": %v\n"
  	a = append(a, err)
  	fmt.Fprintf(os.Stderr, format, a...)
  	os.Exit(exit)
  }
docs: // Err displays a formatting error message to standard error,
  // appending the error string, and exits with the status code from
  // `exit`, à la err(3).