code: func (fs *FlagSet) failf(format string, a ...interface{}) error {
  	err := fmt.Errorf(format, a...)
  	fmt.Fprintln(fs.Out(), err)
  	if os.Args[0] == fs.name {
  		fmt.Fprintf(fs.Out(), "See '%s --help'.\n", os.Args[0])
  	} else {
  		fmt.Fprintf(fs.Out(), "See '%s %s --help'.\n", os.Args[0], fs.name)
  	}
  	return err
  }
docs: // failf prints to standard error a formatted error and usage message and
  // returns the error.