code: @UiThread
      public void notifyParentInserted(int parentPosition) {
          P parent = mParentList.get(parentPosition);
  
          int flatParentPosition;
          if (parentPosition < mParentList.size() - 1) {
              flatParentPosition = getFlatParentPosition(parentPosition);
          } else {
              flatParentPosition = mFlatItemList.size();
          }
  
          int sizeChanged = addParentWrapper(flatParentPosition, parent);
          notifyItemRangeInserted(flatParentPosition, sizeChanged);
      }
docs: Notify any registered observers that the parent reflected at {@code parentPosition}
  has been newly inserted. The parent previously at {@code parentPosition} is now at
  position {@code parentPosition + 1}.
  <p>
  This is a structural change event. Representations of other existing items in the
  data set are still considered up to date and will not be rebound, though their
  positions may be altered.
  
  @param parentPosition Position of the newly inserted parent in the data set, relative
  to the list of parents only.
  
  @see #notifyParentRangeInserted(int, int)