code: public WorkbookWriter turnToSheet(int index) {
      checkElementIndex(index, workbook.getNumberOfSheets());
      sheet = workbook.getSheetAt(index);
      return this;
    }
docs: Turns this {@link WorkbookWriter} to certain sheet. Sheet names can be
  found by {@link #getAllSheetNames}.
  
  @param index
  of a sheet
  @return this {@link WorkbookWriter}