The pipeline
<p:declare-step xmlns:p="http://www.w3.org/ns/xproc" name="pipeline" version="3.0">
<p:output port="result"/>
<p:input port="source">
<document>
<title>Some Title</title>
<para>Some paragraph.</para>
</document>
</p:input>
<p:input port="alternate">
<document>
<para>Some alternate paragraph.</para>
</document>
</p:input>
<p:choose>
<p:when test="//title">
<p:with-input>
<p:pipe step="pipeline" port="alternate"/>
</p:with-input>
<p:output port="result"/>
<p:identity>
<p:with-input port="source">
<p:inline>
<p>Alternate input port contains a //title element.</p>
</p:inline>
</p:with-input>
</p:identity>
</p:when>
<p:when test="//title">
<p:with-input>
<p:pipe step="pipeline" port="source"/>
</p:with-input>
<p:output port="result"/>
<p:identity>
<p:with-input port="source">
<p:inline>
<p>Source input port contains a //title element.</p>
</p:inline>
</p:with-input>
</p:identity>
</p:when>
<p:otherwise>
<p:output port="result"/>
<p:identity>
<p:with-input port="source">
<p:inline>
<p>no input contains a //title element.</p>
</p:inline>
</p:with-input>
</p:identity>
</p:otherwise>
</p:choose>
</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="/p">Root element is not 'p'.</s:assert>
<s:assert test="/p/text()='Source input port contains a //title element.'">Text in 'p' is not 'Source input port contains a //title element.'.</s:assert>
</s:rule>
</s:pattern>
</s:schema>