docs: public final int childItemPosition(int childAdapterPosition) {
          int itemCount = 0;
  
          int parentCount = parentItemCount();
          for (int i = 0; i < parentCount; i++) {
              itemCount += 1;
  
              if (isExpanded(i)) {
                  int childCount = childItemCount(i);
                  itemCount += childCount;
  
                  if (childAdapterPosition < itemCount) {
                      return childCount - (itemCount - childAdapterPosition);
                  }
              } else {
                  // itemCount += 1;
              }
          }
  
          throw new IllegalStateException("The adapter position is invalid: " + childAdapterPosition);
      }