code: func (f *EnumFlag) Set(s string) error {
  	for i := range f.vs {
  		if f.vs[i] == s {
  			f.i = i
  			return nil
  		}
  	}
  	return fmt.Errorf("must be one of %v", f.Type())
  }
docs: // Set sets the enum value. If s is not a valid enum value, an error is
  // returned.