def to_xml_string(str = '')
      str << '<cfRule '
      serialized_attributes str
      str << '>'
      str << ('<formula>' << [*self.formula].join('</formula><formula>') << '</formula>') if @formula
      @color_scale.to_xml_string(str) if @color_scale && @type == :colorScale
      @data_bar.to_xml_string(str) if @data_bar && @type == :dataBar
      @icon_set.to_xml_string(str) if @icon_set && @type == :iconSet
      str << '</cfRule>'
    end
def to_xml_string(str = '')
      str << '<cfRule '
      serialized_attributes str
      str << '>'
      str << ('<formula>' << [*self.formula].join('</formula><formula>') << '</formula>') if @formula
      @color_scale.to_xml_string(str) if @color_scale && @type == :colorScale
      @data_bar.to_xml_string(str) if @data_bar && @type == :dataBar
      @icon_set.to_xml_string(str) if @icon_set && @type == :iconSet
      str << '</cfRule>'
    end
Serializes the conditional formatting rule
 @param [String] str
 @return [String]