code: def writer io_or_path, type=Excel, version=self.version
        if type == Excel
          Excel::Writer::Workbook.new io_or_path
        else
          raise NotImplementedError, "No Writer defined for #{type}"
        end
      end
docs: Returns a new instance of the default Writer class for this Workbook (can
   only be an Excel::Writer::Workbook at this time)