select="item/@set1[. = /items/item/@set2]"
select="item/@set1[not(. = /items/item/@set2)]"
select="item/@set2[not(. = /items/item/@set1)]"

that'll return duplicate entries which can be removed eg by using the
count(.| trick I used on a posting early today (and it's too late to
reconstruct:-)


(untested, and time for bed, use at own risk:-)

David



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