Seems like a long-winded way of just saying you want to check if <class> exists:

<xsl:when test="doc('factbase.xml')/facts/class">

If you then want to do something specific if the @id eq $id then add a
condition inside the xsl:when.

<xsl:when test="doc('factbase.xml')/facts/class[@id eq $id]">
 there is a class with a matching @id
</xsl:when>
<xsl:when test="doc('factbase.xml')/facts/class">
 there are one or more class elements, but none of them a matching @id
</xsl:when>
<xsl:otherwise>
 this aren't any class elements...
</xsl:otherwise>

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