// only one of Prefix, And and Tag should be present in the output.
func (f Filter) MarshalXML(e *xml.Encoder, start xml.StartElement) error {
	if !f.set {
		return nil
	}

	if err := e.EncodeToken(start); err != nil {
		return err
	}

	switch {
	case !f.And.isEmpty():
		if err := e.EncodeElement(f.And, xml.StartElement{Name: xml.Name{Local: "And"}}); err != nil {
			return err
		}

	}

	if len(s.Items) == 0 {
		return nil
	}

	if err := e.EncodeToken(start); err != nil {
		return err
	}

	for _, item := range s.Items {
		if err := e.Encode(xmlKeyEntry{
			XMLName: xxml.Name{Local: item.Key},
			Value:   item.Value,
		}); err != nil {
			return err
		}
	}

	return e.EncodeToken(start.End())
}