Using keys to lookup from current stylesheet

I'm trying to use a key to lookup elements in the current stylesheet. The
AcronymnStandsFor is called from another stylesheet that imports the one
shown below to populate the @title attribute of xhtml:acronym elements.
I've used Michael Kay's advice from his book and used the for-each
select="document('')" to switch the context to the current (stylesheet)
document. However, the key doesn't seem to find anything. I've tested that
the values that are coming in in the $acronym parameter are correct. Can
anybody spot what the problem might be? Thanks.

- Justin

<xsl:stylesheet version="1.0"
    xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
    xmlns:a="http://cde.berkeley.edu/docbook/constant/acronym"
    exclude-result-prefixes="a">
    
    <xsl:key name="AcronymKey" match="a:acronymItem" use="a:acronym"/>
    
    <xsl:template name="AcronymnStandsFor">
        <xsl:param name="acronym"/>
        <!-- change context to current document so the key will work -->
        <xsl:for-each select="document('')">
            <xsl:value-of select="key('AcronymKey',$acronym)/a:standsFor"/>
        </xsl:for-each>
    </xsl:template>
    
    <!-- acronym lookups -->
    <a:acronymList>
        <a:acronymItem>
            <a:acronym>Ant</a:acronym>
            <a:standsFor>Another Neat Tool</a:standsFor>
        </a:acronymItem>
        ...
    </a:acronymList>
</xsl:stylesheet>


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