code: func printDefaultValue(f *flag.Flag) {
  	format := "  -%s=%s: %s\n"
  	if f.DefValue == "" {
  		format = "  -%s=%q: %s\n"
  	}
  	fmt.Fprintf(os.Stderr, format, f.Name, f.DefValue, f.Usage)
  }
docs: // printDefaultValue is a helper function to print out a user friendly
  // usage message of a flag. It's useful since we want to write customized
  // usage message on selected subsets of the global flag set. It is
  // borrowed from standard library source code. Since flag value type is
  // not exported, default string flag values are printed without
  // quotes. The only exception is the empty string, which is printed as "".