def to_xml_string(str = "")
      return if empty?
      str << "<dataValidations count='#{size}'>"
      each { |item| item.to_xml_string(str) }
      str << '</dataValidations>'
    end
def to_xml_string(str = "")
      return if empty?
      str << "<dataValidations count='#{size}'>"
      each { |item| item.to_xml_string(str) }
      str << '</dataValidations>'
    end
serialize the conditional formattings