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