code: public ModelAdapter<?, Item> add(int position, List<Item> items) {
          if (mOriginalItems != null && items.size() > 0) {
              if (mItemAdapter.isUseIdDistributor()) {
                  mItemAdapter.getIdDistributor().checkIds(items);
              }
              mOriginalItems.addAll(getAdapterPosition(mItemAdapter.getAdapterItems().get(position)) - mItemAdapter.getFastAdapter().getPreItemCount(position), items);
              publishResults(mConstraint, performFiltering(mConstraint));
              return mItemAdapter;
          } else {
              return mItemAdapter.addInternal(position, items);
          }
      }
docs: add a list of items at the given position within the existing items
  
  @param position the global position
  @param items    the items to add