Update class_structure with AML?

Hi Everyone,

I'm trying to add new classes to the class_structure of the Part ItemType with AML, but the results are giving me an empty class_structure. 

What I tried is something like 

<AML>
<Item type="ItemType" action='edit' id = "xxx">
<class_structure>
<class id="xxx">
<class name="Component" id="xxx"/>
<class name="Assembly" id="xxx"/>
<class name="Material" id="xxx"/>
<class name="Software" id="xxx"/>
<class name="Phantom" id="xxx" />
<class  name="MBOM only Part" id="xxx"/>

<class  name="Cat" id="xxx"/>
</class>
</class_structure>
</Item>
</AML>

and the result is an empty class_structure.

-<Result>
-<Item type =" ItemType " typeId =" 450906E86E304F55A34B3C0D65C097EA " id =" 4F1AC04A2B484F3ABA4E20DB63808A88 " >
 <allow_private_permission>1</allow_private_permission>
 <auto_search>0</auto_search>
 <class_structure/>
.....
 
Any suggestions of how to correctly update the class_structure would be appreciated.
Kevin