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/>
<doc/>
</p:with-input>
</p:identity>
<p:if test="/true" name="if">
<p:with-input>
<true/>
</p:with-input>
<p:for-each>
<p:add-attribute match="/doc" attribute-name="a" attribute-value="{p:iteration-position()}"/>
</p:for-each>
</p:if>
<p:wrap-sequence wrapper="result"/>
</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="/result">Root element is not 'result'.</s:assert>
<s:assert test="count(/result/*)=2">Root should only have one child.</s:assert>
<s:assert test="/result/*[1]/name()='doc'">First child of 'result' is not 'doc'.</s:assert>
<s:assert test="/result/*[2]/name()='doc'">Second child of 'result' is not 'doc'.</s:assert>
<s:assert test="/result/doc[1]/@a='1'">On first child @a!=1.</s:assert>
<s:assert test="/result/doc[2]/@a='2'">On second child @a!=2.</s:assert>
</s:rule>
</s:pattern>
</s:schema>
