Firefox Development: XUL: nesting trees

I have posted a piece of code on Mozilla Dev site that demonstrates how to nest nodes in the XUL tree. I will also post it here:

<tree id="myTree" flex="1" hidecolumnpicker="false" seltype="single" class="tree">
 <treecols id="myTree2-treeCols">
  <treecol id="myTree2-treeCol0" primary="true" flex="2" label="Column A" persist="width" ordinal="1"/>
  <splitter class="tree-splitter" ordinal="2"/>
  <treecol id="myTree2-treeCol1" flex="1" label="Column B" persist="width" ordinal="3"/>
 </treecols>
 <treechildren>
  <treeitem>
   <treerow>
    <treecell label="1"/>
    <treecell label="a"/>
   </treerow>
  </treeitem>
  <treeitem container="true" open="true"> <!-- Make sure to set container="true" -->
   <treerow>
    <treecell label="2"/>
    <treecell label="b"/>
   </treerow>
   <treechildren>
    <treeitem>
     <treerow>
      <treecell label="2a"/>
      <treecell label="ba"/>
     </treerow>
    </treeitem>
   </treechildren>
  </treeitem>
 </treechildren>
</tree>