code: static OutputStream toXlsxFile(final IDataSet dataSet) {
          ByteArrayOutputStream xlsx = new ByteArrayOutputStream();
          
          try { toWorkbook(dataSet, (Workbook) null).write(xlsx); }
          catch (IOException e) { throw new CalculationEngineException(e); }
          
          return xlsx;
      }
docs: Converts {@link IDataSet} to new {@link Workbook},
  then writes this Workbook to new {@link ByteArrayOutputStream}.