docs: public Element insertChildren(int index, Node... children) {
          Validate.notNull(children, "Children collection to be inserted must not be null.");
          int currentSize = childNodeSize();
          if (index < 0) index += currentSize +1; // roll around
          Validate.isTrue(index >= 0 && index <= currentSize, "Insert position out of bounds.");
  
          addChildren(index, children);
          return this;
      }