The pipeline
<p:declare-step xmlns:p="http://www.w3.org/ns/xproc" xmlns:xs="http://www.w3.org/2001/XMLSchema" version="3.0">
<p:output port="result"/>
<p:option name="select" as="xs:string?" select="'chapter'"/>
<p:variable name="eq" select="'='"/>
<p:identity>
<p:with-input>
<book>
<chapter>Hello, world.</chapter>
<appendix>Goodbye, world.</appendix>
</book>
</p:with-input>
</p:identity>
<p:filter select="/book/*[local-name(.) {$eq} '{$select}']"/>
<p:wrap-sequence wrapper="result"/>
</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:ns prefix="c" uri="http://www.w3.org/ns/xproc-step"/>
<s:pattern>
<s:rule context="/*">
<s:assert test="self::result">The result root is not result.</s:assert>
<s:assert test="chapter">There is no chapter child.</s:assert>
</s:rule>
</s:pattern>
</s:schema>
