Second XSLT using the Muenchian method (I)

gistfile1.xml

<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
    
    <xsl:param name="ticket" select="__none__" />
    <xsl:output encoding="utf-8" method="xml" indent="yes" />

    <xsl:template match="log">
        <xsl:copy>
            <xsl:element name="ticket">
                <xsl:value-of select="$ticket" />
            </xsl:element>
            <xsl:apply-templates select="logentry[msg[contains(text(), $ticket)]]" />
        </xsl:copy>
    </xsl:template>

    <xsl:template match="logentry">
        <xsl:copy-of select="."/>
    </xsl:template>

</xsl:stylesheet>