key() in an XSLT 2.0 pattern must have exactly two arguments

<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet
     xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
     version="2.0">

<xsl:key name="maindoc" match="person" use="@firstname"/>
<xsl:variable name="doc2" select="/"/>
<xsl:template match="key('maindoc','Sal',$doc2)">
     <xsl:value-of select="@firstname"/>
</xsl:template>

</xsl:stylesheet>

how come?

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