def to_xml_string(str="")
      str << '<?xml version="1.0" encoding="UTF-8"?>'
      str << ('<comments xmlns="' << XML_NS << '"><authors>')
      authors.each do  |author|
        str << ('<author>' << author.to_s << '</author>')
      end
      str << '</authors><commentList>'
      each do |comment|
        comment.to_xml_string str
      end
      str << '</commentList></comments>'

    end
def to_xml_string(str="")
      str << '<?xml version="1.0" encoding="UTF-8"?>'
      str << ('<comments xmlns="' << XML_NS << '"><authors>')
      authors.each do  |author|
        str << ('<author>' << author.to_s << '</author>')
      end
      str << '</authors><commentList>'
      each do |comment|
        comment.to_xml_string str
      end
      str << '</commentList></comments>'

    end
serialize the object
 @param [String] str
 @return [String]