public void addLast(PDOutlineItem newChild)
    {
        requireSingleNode(newChild);
        append(newChild);
        updateParentOpenCountForAddedChild(newChild);
    }
@Test
    void cannotAddLastAList()
    {
        PDOutlineItem child = new PDOutlineItem();
        child.insertSiblingAfter(new PDOutlineItem());
        child.insertSiblingAfter(new PDOutlineItem());
        assertThrows(IllegalArgumentException.class, () -> root.addLast(child));
    }