func (tag *Tag) UnmarshalXML(d *xml.Decoder, start xml.StartElement) error {
	type subTag Tag  
	var st subTag
	if err := d.DecodeElement(&st, &start); err != nil {
		return err
	}
	if err := Tag(st).Validate(); err != nil {
		return err
	}
	*tag = Tag(st)
	return nil
}
UnmarshalXML decodes XML data to tag.
package tags
import (
	"encoding/xml"
	"io"
	"net/url"
	"strings"
	"unicode/utf8"
)
type Error interface {
	error
	Code() string
}
type errTag struct {
	code    string
	message string
}
func (err errTag) Code() string {
	return err.code
}
func (err errTag) Error() string {
	return err.message
}
var (
	errTooManyObjectTags = &errTag{"BadRequest", "Tags cannot be more than 10"}
	errTooManyTags       = &errTag{"BadRequest", "Tags cannot be more than 50"}
	errInvalidTagKey     = &errTag{"InvalidTag", "The TagKey you have provided is invalid"}
	errInvalidTagValue   = &errTag{"InvalidTag", "The TagValue you have provided is invalid"}
	errDuplicateTagKey   = &errTag{"InvalidTag", "Cannot provide multiple Tags with the same key"}
)
const (
	maxKeyLength      = 128
	maxValueLength    = 256
	maxObjectTagCount = 10
	maxTagCount       = 50
)
func checkKey(key string) error {
	if len(key) == 0 || utf8.RuneCountInString(key) > maxKeyLength || strings.Contains(key, "&") {
		return errInvalidTagKey
	}
	return nil
}
func checkValue(value string) error {
	if utf8.RuneCountInString(value) > maxValueLength || strings.Contains(value, "&") {
		return errInvalidTagValue
	}
	return nil
}
type Tag struct {
	Key   string `xml:"Key"`
	Value string `xml:"Value"`
}
func (tag Tag) String() string {
	return tag.Key + "=" + tag.Value
}
func (tag Tag) IsEmpty() bool {
	return tag.Key == ""
}
func (tag Tag) Validate() error {
	if err := checkKey(tag.Key); err != nil {
		return err
	}
	return checkValue(tag.Value)
}
func (tag Tag) MarshalXML(e *xml.Encoder, start xml.StartElement) error {
	if err := tag.Validate(); err != nil {
		return err
	}
	type subTag Tag  
	return e.EncodeElement(subTag(tag), start)
}