Visual DOM and apply and partialdiff example of difficulties

 From the table
http://www.w3.org/TR/MathML2/appendixd.html#dom.elementTable
it can be seen that many elements have the MathMLPredefinedSymbol 
<http://www.w3.org/TR/MathML2/appendixd.html#dom.PredefinedSymbol> 
interface.  Maybe the short question and answer would be: is it intended 
that an implementor should extend the MathMLPredefinedSymbol 
<http://www.w3.org/TR/MathML2/appendixd.html#dom.PredefinedSymbol> for 
each associated element?
then get all it's siblings that define it from the parent apply?
Or extend apply for each possible fiirst operation child(treating it 
like an attribute)?
O many cases under MathMLApplyElement?

Take
http://www.w3.org/TR/MathML2/chapter4.html#contm.partialdiff
for example.
The number of bvars and their degrees effect the presentation 
considerably.  Under what interface would the presentation be recommended?

Any ideas?  Considerations?

-Roger