code: func YesOrNoField(f Field) Field {
  	f.Type = FieldChoice
  	f.ChoiceVariants = []string{"yes", "no"}
  	f.Validator = func(v string) error {
  		var x YesOrNo
  		return x.Set(v)
  	}
  	return f
  }
docs: // YesOrNoField modifies the field so that it corresponds to YesOrNo value.
  //
  // It sets 'Type', 'ChoiceVariants' and 'Validator' properties.