Nodesets handling (getting subsets)

I have a nodeset parameter called $selected-slides. It is a set of
'slide' elements with a 'number' attribute.

I would like to remove from that nodeset those elements whose 'number'
attribute is not associated with any draw:page element in the xml
source. In other words, those 'slide' elements whose 'number' attribute
is not the position number of any draw:page element in the source xml.

I get no $selected-slides elements because (IMHO) @number does not refer
 to the 'slide' element inside $selected-slides.
How could I solve this problem?

Thanks in advance,
Marco