regards,
Garvin

<xsl:param name="id" />

<xsl:template match="your-start">
   <xsl:apply-templates match="doc('factbase.xml')/facts/class" />
</xsl:template>

<xsl:template match="class[@id = $id]">
   ...do your stuff with a matching $id=@id for class-nodes...
</xsl:template>

<xsl:template match="facts[not(class/@id = $id)]/class">
...do your stuff when no @id matches the supplied $id, but class-nodes exist...
</xsl:template>

<xsl:template match="facts[empty(class)]">
  ...do your stuff when there are no class-nodes...
</xsl:template>