The pipeline
<p:declare-step xmlns:map="http://www.w3.org/2005/xpath-functions/map" xmlns:p="http://www.w3.org/ns/xproc" xmlns:xs="http://www.w3.org/2001/XMLSchema" version="3.0">
<p:output port="result"/>
<p:identity>
<p:with-input>
<p:inline document-properties="map{'serialization' : map{xs:QName('indent') : true() }}">
<doc/>
</p:inline>
</p:with-input>
</p:identity>
<p:identity>
<p:with-input>
<result>{map:get(p:document-property(., xs:QName('serialization')), xs:QName('indent'))}</result>
</p:with-input>
</p:identity>
</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="result">The document root is not result.</s:assert>
<s:assert test="result/text()='true'">Text node of result is not 'true'.</s:assert>
</s:rule>
</s:pattern>
</s:schema>
