def to_xml_string(str = "")
      str << ('<c:' << @tag_name.to_s << '>')
      str << ('<c:ptCount val="' << @pt.size.to_s << '"/>')
      @pt.each_with_index do |value, index|
        value.to_xml_string index, str
      end
      str << ('</c:' << @tag_name.to_s << '>')
    end
def to_xml_string(str = "")
      str << ('<c:' << @tag_name.to_s << '>')
      str << ('<c:ptCount val="' << @pt.size.to_s << '"/>')
      @pt.each_with_index do |value, index|
        value.to_xml_string index, str
      end
      str << ('</c:' << @tag_name.to_s << '>')
    end
serialize the object