code: public void insert(final int index, @NonNull final T item) {
          if (mActiveIndexes.isEmpty() && mPendingItemsToInsert.isEmpty()) {
              mActiveIndexes.add(new AtomicInteger(index));
              //noinspection unchecked
              mInsertable.add(index, item);
          } else {
              mPendingItemsToInsert.add(new Pair<>(index, item));
          }
      }
docs: Insert an item into the queue at given index. Will directly call {@link Insertable#add(int,
  Object)} if there are no active index-item pairs. Otherwise, the pair will be queued.
  
  @param index the index at which the item should be inserted.
  @param item  the item to insert.