lib/spreadsheet/workbook.rb
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
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
Returns a new instance of the default Writer class for this Workbook (can
 only be an Excel::Writer::Workbook at this time)