docs: public int mergeToBackingIndex(@NonNull ObservableList<? extends T> backingList, int index) {
          if (index < 0) {
              throw new IndexOutOfBoundsException();
          }
          int size = 0;
          for (int i = 0, listsSize = lists.size(); i < listsSize; i++) {
              List<? extends T> list = lists.get(i);
              if (backingList == list) {
                  if (index < list.size()) {
                      return size + index;
                  } else {
                      throw new IndexOutOfBoundsException();
                  }
              }
              size += list.size();
          }
          throw new IllegalArgumentException();
      }