code: @Override
      public XlsWorksheet getSheet(int i)
      {
          XlsWorksheet ret = null;
          if(workbook != null)
              ret = new XlsWorksheet(workbook.getSheet(i));
          else if(writableWorkbook != null)
              ret = new XlsWorksheet(writableWorkbook.getSheet(i));
          return ret;
      }
docs: Returns the worksheet at the given index in the workbook.
  @param i The index of the worksheet
  @return The worksheet at the given index in the workbook