def to_xml(indent=4)
      xml =''
      formatter = REXML::Formatters::Pretty.new(indent)
      formatter.write(@doc.root, xml)
      xml
    end
def to_xml(indent=4)
      xml =''
      formatter = REXML::Formatters::Pretty.new(indent)
      formatter.write(@doc.root, xml)
      xml
    end
return xml as string format
 @param [Integer] indent size(bytes)
 @return [String] raw xml string