code: func defaultUsage(f *FlagSet) {
  	if f.name == "" {
  		fmt.Fprintf(f.out(), "Usage:\n")
  	} else {
  		fmt.Fprintf(f.out(), "Usage of %s:\n", f.name)
  	}
  	f.PrintDefaults()
  }
docs: // defaultUsage is the default function to print a usage message.