xsl:variable and key

I want to assign a node-set returned by key() to a variable.

This works fine : <xsl:variable name="foo" select=3D"key('XXX', YYY)"/>

But now I have a condition and I have to use <xsl:choose>. And in such case 
I am not able to set the variable.
The pseudo for what I want is something like this:
<xsl:variable name="foo">
 <xsl:choose>
    <xsl:when test="SOME-TEST">
         SELECT node-set returned by key('XXX', YYY)
    </xsl:when>
    <xsl:otherwise>
          SELECT node-set ancestor::*[1]
    <xsl:otherwise>
 </xsl:choose>
</xsl:variable>

Any suggesstions? Thanks.

Sameer.

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