Help with nested elements

[Andrea Chiumenti]

Hello I'd like to create something similar to this:

<item id="a">
   <item id="a-1">
       <item id="a-1-1"/>
       <item id="a-1-2"/>
       <item id="a-1-3"/>
   </item>
   <item id="a-2">
       <item id="a-2-1"/>
       <item id="a-2-2"/>
   </item>
</item>

I've defined this, but It's wrong:
      <element name="item">
           <attribute name="id" required="true"/>
           <attribute name="parentId" required="false"/>
           <attribute name="order" required="false"/>
           <attribute name="label" required="false"/>
           <element name="item">
               <rules>
                   <invoke-parent method="addNode"/>
               </rules>
           </element>
           <conversion class="MyItem" />
       </element>

Can someone please tell me how to do ?

Thanks,
kiuma