def to_xml_string(str = '')
      str << '<border '
      serialized_attributes str
      str << '>'
      # enforces order
      [:start, :end, :left, :right, :top, :bottom, :diagonal, :vertical, :horizontal].each do |k|
        @prs.select { |pr| pr.name == k }.each do |part|
          part.to_xml_string(str)
        end
      end
      str << '</border>'
    end
def to_xml_string(str = '')
      str << '<border '
      serialized_attributes str
      str << '>'
      # enforces order
      [:start, :end, :left, :right, :top, :bottom, :diagonal, :vertical, :horizontal].each do |k|
        @prs.select { |pr| pr.name == k }.each do |part|
          part.to_xml_string(str)
        end
      end
      str << '</border>'
    end
Serializes the object
 @param [String] str
 @return [String]