code: public Iterable<List<String>> toLists() {
      checkState(!isClosed, WORKBOOK_CLOSED);
      Iterable<List<String>> listsIterable = Iterables.transform(sheet, item -> {
        return rowToList(item);
      });
      return hasHeader ? Iterables.skip(listsIterable, 1) : listsIterable;
    }
docs: Converts the spreadsheet to String Lists by a List Iterable.
  
  @return List of String Iterable