[ differential update ] - How to ?

hello,
I've 2 questions :

1 - I'm looking for a method to update an XML Tree with PL/SQL (and/ or) XSLT... but this update must be differential... 2 - If I use triggers at update time how can I get the old node fragment (like :OLD operator in standard SQL ??) after update...

any idea to do this ??

Thanks for your help...
regards.

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JH