The pipeline
<p:declare-step xmlns:p="http://www.w3.org/ns/xproc" version="3.0">
<p:output port="result"/>
<p:insert match="child::div/p[3]" position="after">
<p:with-input port="source">
<div>
<p>First paragraph.</p>
<p>Middle paragraph.</p>
<p>Last paragraph.</p>
</div>
</p:with-input>
<p:with-input port="insertion">
<p:inline>
<p>New after paragraph.</p>
</p:inline>
</p:with-input>
</p:insert>
</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="div">The document root is not 'div'.</s:assert>
<s:assert test="child::div/p[1]/text()='First paragraph.'">The text of the first p is not 'First paragraph'.</s:assert>
<s:assert test="child::div/p[2]/text()='Middle paragraph.'">The text of the second p is not 'Middle paragraph'.</s:assert>
<s:assert test="child::div/p[3]/text()='Last paragraph.'">The text of the third p is not 'Last paragraph'.</s:assert>
<s:assert test="child::div/p[4]/text()='New after paragraph.'">The text of the last p is not 'New after paragraph'.</s:assert>
<s:assert test="count(/div/p)=4">Root element does not have exactly four p-children.</s:assert>
</s:rule>
</s:pattern>
</s:schema>
