making grandparents of grandchildren

Dear listmembers, 

Beg your pardon for my ignorance. Just starting to deal with XSLT. 
I have to transform a list pictures grouped by their authors into
another list grouped by their owners. This would probably make a good
tutorial example. 

The structure of the first list is 

<authorgroup>
<name></name>
<picture>
<title></title>
<owner></owner>
<inv></inv>
</picture>
<picture>
...
</picture>
</authorgroup> 
<authorgroup>
...
</authorgroup>


What I tried for some time to achieve in vain, is group paintings
belonging to same owners under their names sorted and add author names
to each picture in the list. Would really appreciate your help (or our
exhibition in Madrid will have to be postponed :).

Best wishes

Piotr
 

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