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