The pipeline
<p:declare-step xmlns:p="http://www.w3.org/ns/xproc" name="pipeline" version="3.0">
<p:output port="result"/>
<p:variable name="var" select="42"/>
<p:identity name="producer">
<p:with-input>
<doc/>
</p:with-input>
</p:identity>
<p:identity>
<p:with-input>
<error/>
</p:with-input>
</p:identity>
<p:choose>
<p:when test="false()">
<p:identity>
<p:with-input>
<error/>
</p:with-input>
</p:identity>
</p:when>
<p:otherwise>
<p:add-attribute attribute-name="att" attribute-value="{$var}">
<p:with-input pipe="@producer"/>
</p:add-attribute>
</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="doc">The pipeline root is not doc.</s:assert>
<s:assert test="doc/@att=42">Attribute @att is not 42.</s:assert>
</s:rule>
</s:pattern>
</s:schema>
