code: public static XlsWorkbook createWorkbook(OutputStream os, Workbook existing) 
          throws IOException
      {
          try
          {
              if(existing != null)
                  return new XlsWorkbook(jxl.Workbook.createWorkbook(os, 
                      (jxl.Workbook)existing.getWorkbook(), settings));
              else
                  return new XlsWorkbook(jxl.Workbook.createWorkbook(os, settings));
          }
          catch(jxl.read.biff.BiffException e)
          {
              throw new IOException(e);
          }
      }
docs: Creates a new workbook object.
  @param os The output stream for the workbook
  @param existing An existing workbook to add to
  @return The new workbook object
  @throws IOException if the workbook cannot be written