It requires the following features: xslt-1.
The pipeline
<p:declare-step xmlns:p="http://www.w3.org/ns/xproc" version="3.0">
<p:output port="result"/>
<p:variable name="node" select=".">
<value>42</value>
</p:variable>
<p:xslt version="1.0" parameters="map{'value' : $node}">
<p:with-input>
<doc/>
</p:with-input>
<p:with-input port="stylesheet">
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="1.0">
<xsl:param name="value"/>
<xsl:template match="doc">
<answer>
<xsl:value-of select="$value"/>
</answer>
</xsl:template>
</xsl:stylesheet>
</p:with-input>
</p:xslt>
</p:declare-step>
Schematron validation
<s:schema xmlns="http://www.w3.org/1999/xhtml" xmlns:s="http://purl.oclc.org/dsdl/schematron" queryBinding="xslt2">
<s:pattern>
<s:rule context="/">
<s:assert test="/answer">The document root is not 'answer'.</s:assert>
<s:assert test="/answer/text()='42'">The answer is not '42'.</s:assert>
</s:rule>
</s:pattern>
</s:schema>
