The pipeline
<p:declare-step xmlns:p="http://www.w3.org/ns/xproc" version="3.0">
<p:output port="result"/>
<p:viewport match="para" name="viewport">
<p:with-input>
<doc>
<para>Some paragraph.</para>
<para>Some paragraph.</para>
<para>Some paragraph.</para>
<para>
<para>Nested paragraph.</para>
</para>
</doc>
</p:with-input>
<p:identity>
<p:with-input port="source">
<p:inline>
<foo/>
</p:inline>
</p:with-input>
</p:identity>
</p:viewport>
<p:identity>
<p:with-input port="source">
<p:pipe step="viewport" port="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="doc">The document root is not 'doc'.</s:assert>
<s:assert test="count(doc/*)=4">The document root does not have four children.</s:assert>
<s:assert test="count(doc/foo)=4">The document root does not have four children 'foo'.</s:assert>
</s:rule>
</s:pattern>
</s:schema>
