def to_xml_string(str = '')
      str << '<dxf>'
      # Dxf elements have no attributes. All of the instance variables
      # are child elements.
      CHILD_ELEMENTS.each do |element|
        self.send(element).to_xml_string(str) if self.send(element)
      end
      str << '</dxf>'
    end
def to_xml_string(str = '')
      str << '<dxf>'
      # Dxf elements have no attributes. All of the instance variables
      # are child elements.
      CHILD_ELEMENTS.each do |element|
        self.send(element).to_xml_string(str) if self.send(element)
      end
      str << '</dxf>'
    end
Serializes the object
 @param [String] str
 @return [String]