[xsl] union/intersection on attribute values

[Dan Diebolt <dandiebolt@xxxxxxxxx>]

Given the following:

<items>
 <item id="1" set1="A"/>
 <item id="2" set1="B"/>
 <item id="3" set1="C"/>
 <item id="4" set2="A"/>
 <item id="5" set2="C"/>
 <item id="6" set2="D"/>
</items>

How do you determine the *attributes*values* that are:

1) represented in both set1 and set2 - A and C
2) represented in set1 but not set2 - B
3) represented in set2 but not set1 - D

By determined, lets say I want to process the attribute values
with <xsl:for-each select="?">

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