p:choose-012(AB)

The pipeline

<p:declare-step xmlns:err="http://www.w3.org/ns/xproc-error" xmlns:p="http://www.w3.org/ns/xproc" version="3.0">
   <p:output port="result"/>
   <p:choose>
      <p:when test="true()" name="when">
         <p:identity>
            <p:with-input>
               <doc1/>
            </p:with-input>
         </p:identity>
      </p:when>
      <p:when test="true()">
         <p:identity>
            <p:with-input pipe="@when"/>
         </p:identity>
      </p:when>
   </p:choose>
</p:declare-step>