code: @Override
      public void appendToSheet(FileColumn[] columns, List<String[]> lines, String sheetName)
          throws IOException
      {
          try
          {
              XlsWorksheet sheet = getSheet(sheetName);
              if(sheet != null)
                  appendRows((WritableSheet)sheet.getSheet(), columns, lines, sheetName);
          }
          catch(WriteException e)
          {
              throw new IOException(e);
          }
      }
docs: Adds the given lines of data to an existing sheet in the workbook.
  @param columns The column definitions for the worksheet
  @param lines The list of lines to be added to the worksheet
  @param sheetName The name of the worksheet to be added
  @throws IOException if the data cannot be appended