code: public void addChild(TreeElement child)
              throws IllegalArgumentException
      {
  
          TreeElement theChild = child;
          theChild.setParent(this);
          if (getName() == null) {
              setName("0");
          }
          if (_children == null) {
              _children = new ArrayList();
          }
          //int n = _children.size();
          //if (n > 0) {
          //    TreeElement node = (TreeElement) _children.get(n - 1);
          //    node.setLast(false);
          //}
          //theChild.setLast(true);
          _children.add(child);
          int n = _children.size();
          theChild.updateName(this, n - 1);
      }
docs: Add a new child node to the end of the list.
  @param child The new child node
  @throws IllegalArgumentException if the name of the new child
  node is not unique