code: function(childNode) {
          if (this.hasChildren()) {
              var sib = this.children[this.children.length - 1];
              sib.nextSibling = childNode;
              childNode.previousSibling = sib;
          }
          this.children[this.children.length] = childNode;
          childNode.applyParent(this);
  
          // part of the IE display issue workaround. If child nodes
          // are added after the initial render, and the node was
          // instantiated with expanded = true, we need to show the
          // children div now that the node has a child.
          if (this.childrenRendered && this.expanded) {
              this.getChildrenEl().style.display = "";
          }
  
          return childNode;
      }
docs: Appends a node to the child collection.
  @method appendChild
  @param childNode {Node} the new node
  @return {Node} the child node
  @private