It requires the following features: p-count or p-count-limit.
The pipeline
<p:declare-step xmlns:p="http://www.w3.org/ns/xproc" name="main" version="3.0">
<p:option name="limit" select="0"/>
<p:output port="result"/>
<p:identity name="id1">
<p:with-input port="source">
<p:inline>
<doc/>
</p:inline>
</p:with-input>
</p:identity>
<p:identity name="id2">
<p:with-input port="source">
<p:inline>
<doc/>
</p:inline>
</p:with-input>
</p:identity>
<p:identity name="id3">
<p:with-input port="source">
<p:inline>
<doc/>
</p:inline>
</p:with-input>
</p:identity>
<p:count limit="{$limit}">
<p:with-input port="source" pipe="result@id1 result@id2 result@id3"/>
</p:count>
</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:ns prefix="c" uri="http://www.w3.org/ns/xproc-step"/>
<s:pattern>
<s:rule context="/*">
<s:assert test="self::c:result">The pipeline root is not c:result.</s:assert>
<s:assert test="string(.) = '1'">The result isn't "1"</s:assert>
</s:rule>
</s:pattern>
</s:schema>
