code: public ListMultimap<String, List<String>> toMultimap() {
      ListMultimap<String, List<String>> content = ArrayListMultimap.create();
  
      String currentSheet = getCurrentSheetName();
      boolean currentHeader = hasHeader;
  
      for (String name : getAllSheetNames()) {
        turnToSheet(name);
        withoutHeader();
        for (List<String> row : toLists()) {
          content.put(name, row);
        }
      }
  
      turnToSheet(currentSheet);
      hasHeader = currentHeader;
  
      return content;
    }
docs: Returns a {@link ListMultimap} which represents the content of this
  workbook. Each sheet name is used as the key, and the value is a Collection
  of String List which contains all fields of a row.
  
  @return {@link ListMultimap}{@literal <String, List<String>>}