The pipeline
<p:declare-step xmlns:c="http://www.w3.org/ns/xproc-step" xmlns:p="http://www.w3.org/ns/xproc" version="3.0">
<p:output port="result"/>
<p:identity>
<p:with-input select="*/text()">
<doc>
<!-- A comment -->
<element1 att="att"/>
<?pi target?>
A text node
<element2/>
</doc>
</p:with-input>
</p:identity>
<p:wrap-sequence wrapper="c: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="count(*) = 1">Document does not have exactly one child element.</s:assert>
<s:assert test="*[1]/name()='c:result'">Root element is not named 'c:result'.</s:assert>
</s:rule>
<s:rule context="/c:result">
<s:assert test="count(node())=count(text())">c:result does not have only text node children.</s:assert>
<s:assert test="normalize-space(string-join(text(),''))='A text node'">The normalized text is not 'A next node'.</s:assert>
</s:rule>
</s:pattern>
</s:schema>
