Testing against node-set

I wanna select all rows from the second rowSet, whose
parentIDs are not equal to any row id from the first
rowSet. In this example, it'd be rows 3 and 6:

        <rowSet id="1">
                <row id="1"/>
                <row id="2"/>
        </rowSet>

        <rowSet id="2">
                <row id="3"/>
                <row id="4">
                        <parentID>1</parentid>
                </row>
                <row id="5">
                        <parentID>2</parentid>
                </row>
                <row id="6">
                        <parentID>3</parentid>
                </row>
        </rowSet>


I'm trying the following code, but it's not working.
All rows from the second rowSet are selected.

<xsl:variable name="firstRowSet"
select="rowSet[(_at_)id=1]/row"/>
<xsl:variable name="secondRowSet"
select="rowSet[(_at_)id=2]/row"/>

<xsl:for-each
select="$secondRowSet[parentID!=$firstRowSet/@id]">
...

Any ideas?


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