[xsl] Comparing values in lists

[tom tom <tomxsllist@xxxxxxxxxxx>]

I have 2 variables containing objects, each with an id, for example:
 
<xsl:variable name="list1">
<object id="1"/>
<object id="2"/>
<object id="3"/>
<object id="4"/>
<object id="5"/>
<object id="6"/>
</xsl:variable>
 
<xsl:variable name="list2">
<object id="1"/>
<object id="3"/>
<object id="5"/>
</xsl:variable>
 
I need to display the objects in list 1 whose ids are not used by any object
in list 2, how is this most easily acheived in XSLT 2?