XSLT is a bit funny in that

<xsl:stylesheet version="1.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:template match="/">
    <xsl:apply-templates select="$var"/>
</xsl:template>
</xsl:stylesheet>

won't run by itself but will execute perfectly well provided it is
imported/included into a stylesheet that is within a hierachy of
stylesheets where one of them defines $var... but it provides no
indication of that fact.

To obfuscate things further, if two definitions of $var exist in
different stylesheets then import precedence comes into play where the
instance of $var to use is decided by which of the import statements
comes last in the importing stylesheet.

Theres plenty of scope to write some hard-to-follow code...