You can simply devise an identity transformation in the xsl documnent and an extinction transformation - or nothing at all,say - herein-below, whereby when the root element's name or local-name match the target name, invoke the identity transformation, otherwise invoke the extinction transformation for parallelism. I use for the demo a param to store the target root local-name.
[tt]
<xsl:stylesheet version="1.0" xmlns:xsl="[ignore]
[/ignore]">
<xsl

utput method="xml" indent="yes" encoding="utf-8" omit-xml-declaration="yes" />
<xsl:template match="@*|node()" mode="copy">
<xsl:copy>
<xsl:apply-templates select="@*|node()" mode="copy" />
</xsl:copy>
</xsl:template>
<xsl:template match="@*|node()" mode="extinct">
</xsl:template>
<xsl:template match="/*">
<xsl:choose>
<xsl:when test="local-name()=$targetroot">
<xsl:apply-templates select="." mode="copy" />
</xsl:when>
<xsl

therwise>
<xsl:apply-templates select="." mode="extinct" />
</xsl

therwise>
</xsl:choose>
</xsl:template>
</xsl:stylesheet>
[/tt]