def to_xml_string(str = '')
      str << '<?xml version="1.0" encoding="UTF-8"?>'
      str << ('<Types xmlns="' << XML_NS_T << '">')
      each { |type| type.to_xml_string(str) }
      str << '</Types>'
    end
def to_xml_string(str = '')
      str << '<?xml version="1.0" encoding="UTF-8"?>'
      str << ('<Types xmlns="' << XML_NS_T << '">')
      each { |type| type.to_xml_string(str) }
      str << '</Types>'
    end
Serializes the object
 @param [String] str
 @return [String]