docs: public final int parentItemPosition(int adapterPosition) {
          int itemCount = 0;
          for (int i = 0; i < parentItemCount(); i++) {
              itemCount += 1;
  
              if (isExpanded(i)) {
                  int childCount = childItemCount(i);
                  itemCount += childCount;
              }
              if (adapterPosition < itemCount) {
                  return i;
              }
          }
  
          throw new IllegalStateException("The adapter position is not a parent type: " + adapterPosition);
      }