The pipeline
<p:declare-step xmlns:p="http://www.w3.org/ns/xproc" version="3.0">
<p:output port="result"/>
<p:identity>
<p:with-input>
<doc a="0"/>
</p:with-input>
</p:identity>
<p:choose>
<p:when test="/doc/@a=1">
<p:with-input>
<doc/>
</p:with-input>
<p:add-attribute match="/doc" attribute-name="a" attribute-value="1"/>
</p:when>
<p:otherwise>
<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="/doc">Root element is not 'doc'.</s:assert>
<s:assert test="/doc/@a='0'">Root has no att 'a' or a!=0.</s:assert>
</s:rule>
</s:pattern>
</s:schema>
