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