For the specific reasons of performance or editing peculiarities Serna introduces several extensions. To make your stylesheet ignore the extensions use the following construct:
<xsl:param name="use-serna-extensions"
select="contains(system-property('xsl:vendor'), 'Syntext')"/>
</xsl:stylesheet>
....
<xsl:if test="$use-serna-extensions">
...
</xsl:if>
The Serna extension elements are preceded with the namespace xmlns:se="http://syntext.com/XSL/Format-1.0" . But for performance reasons Serna treats namespace prefix se conforming to the above-mentioned URL without checking its definition. Make sure you use se namespace for Serna extensions.