RDF Containers in XUL Trees

Hi all,

I had intended to use my own class to describe containers and produce a 
XUL tree, for example:

-----

<my:Container rdf:about="level1" my:name="Parent">
   <rdf:li rdf:resource="level2"/>
</my:Container>

<my:Container rdf:about="level2" my:name="Child">
   <rdf:li rdf:resource="level3"/>
</my:Container>

<my:Container rdf:about="level3" my:name="Sub-Child">
   <rdf:li rdf:resource="level4"/>
</my:Container>

-----

1. What causes templated trees to create twisty children? I had assumed 
it was any declared container, and that I could use <rdf:instanceOf> and 
declare my:Container as:

<rdf:instanceOf
rdf:resource="http://www.w3.org/1999/02/22-rdf-syntax-ns#Seq"/>

But I can't get the tree to build any element unless I use Seq collections


2. I had intended to use my own class collection to be more semantically 
  correct, and allow me to test for a resource having that class property.

Are there any disadvantages from Mozilla's point of view of not just 
using vanilla <rdf:Seq>, <rdf:li>, <rdf:Description> combinations to 
achieve the same thing?

I though I read somewhere that some of the XPCOM methods like 
GetSources() only worked properly with standard rdf containers?

Thoughts much appreciated.

Neil