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:choose name="choose">
<p:when test="p:iteration-position() = 1">
<p:identity>
<p:with-input>
<p:inline>
<first/>
</p:inline>
<p:pipe step="viewport" port="current"/>
</p:with-input>
</p:identity>
</p:when>
<p:when test="p:iteration-position() = p:iteration-size()">
<p:identity>
<p:with-input>
<p:pipe step="viewport" port="current"/>
<p:inline>
<last/>
</p:inline>
</p:with-input>
</p:identity>
</p:when>
<p:otherwise>
<p:identity/>
</p:otherwise>
</p:choose>
</p:viewport>
</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/*)=6">Document root does not have 6 children.</s:assert>
<s:assert test="doc/*[1]/name()='first'">First child is not 'first'.</s:assert>
<s:assert test="doc/*[2]/name()='para'">Second child is not 'para'.</s:assert>
<s:assert test="doc/*[3]/name()='para'">Third child is not 'para'.</s:assert>
<s:assert test="doc/*[4]/name()='para'">Fourth child is not 'para'.</s:assert>
<s:assert test="doc/*[5]/name()='para'">Fifth child is not 'para'.</s:assert>
<s:assert test="doc/*[6]/name()='last'">Sixth child is not 'last'.</s:assert>
</s:rule>
</s:pattern>
</s:schema>
