Testing p:label-elements.
Test ab-label-elements-012.xml is expected to pass.
<p:declare-step xmlns:p="http://www.w3.org/ns/xproc" version="3.0">
   <p:output port="result">
   <p:input port="source">
      <doc>
         <p att="val1">
         <p/>
         <p att="val2">
      </doc>
   </p:input>
   <p:label-elements match="p" attribute="att" replace="false">
</p:declare-step>
<s:schema xmlns:s="http://purl.oclc.org/dsdl/schematron" queryBinding="xslt2">
   <s:pattern>
      <s:rule context="/">
         <s:assert test="doc/p[1]/@att='val1'">Value of doc/p[1]/@att is not 'val1'.</s:assert>
         <s:assert test="doc/p[2]/@att='_2'">Value of doc/p[2]/@att is not '_2'.</s:assert>
         <s:assert test="doc/p[3]/@att='val2'">Value of doc/p[3]/@att is not 'val2'.</s:assert>
      </s:rule>
   </s:pattern>
</s:schema>
Added attribute 'queryBinding' to schematron's schema.
Additional test with replace="false"