The pipeline
<p:declare-step xmlns:p="http://www.w3.org/ns/xproc" xmlns:xs="http://www.w3.org/2001/XMLSchema" version="3.0">
<p:output port="result"/>
<p:input port="source">
<p:inline>
<doc>
<para id="x1">Some paragraph.</para>
<para id="x2">Some paragraph.</para>
<para id="x3">Some paragraph.</para>
</doc>
</p:inline>
</p:input>
<p:for-each name="loop">
<p:with-input select="/doc/para"/>
<p:variable name="q" select="/para/@id" as="xs:string">
<p:pipe step="loop" port="current"/>
</p:variable>
<p:choose>
<p:when test="$q = 'x2'">
<p:identity/>
</p:when>
<p:otherwise>
<p:identity>
<p:with-input port="source">
<p:empty/>
</p:with-input>
</p:identity>
</p:otherwise>
</p:choose>
</p:for-each>
<p:wrap-sequence wrapper="newdoc"/>
</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="newdoc">The document root is not 'newdoc'.</s:assert>
<s:assert test="newdoc/para">The root element does not have a child 'para'</s:assert>
<s:assert test="count(newdoc/para)=1">The root element does not have ONE child 'para'.</s:assert>
<s:assert test="newdoc/para/@id='x2'">The para element with @id='x2' was not selected.</s:assert>
<s:assert test="newdoc/para='Some paragraph.'">The content of para element is not 'Some pharagraph.'</s:assert>
</s:rule>
</s:pattern>
</s:schema>
