code: func (fs *FlagSet) Set(name, value string) error {
  	flag, ok := fs.formal[name]
  	if !ok {
  		return fmt.Errorf("no such flag -%v", name)
  	}
  	if err := flag.Value.Set(value); err != nil {
  		return err
  	}
  	if fs.actual == nil {
  		fs.actual = make(map[string]*Flag)
  	}
  	fs.actual[name] = flag
  	return nil
  }
docs: // Set sets the value of the named flag.