Step available 009 (AB)

The pipeline

<p:declare-step xmlns:ex="http://example.com/ns/xproc-extensions" xmlns:p="http://www.w3.org/ns/xproc" name="main" version="3.0">
   <p:output port="result"/>
   <p:declare-step type="ex:step1">
      <p:input port="source"/>
      <p:output port="result"/>
      <p:declare-step type="ex:step2">
         <p:input port="source"/>
         <p:output port="result"/>
         <p:identity/>
      </p:declare-step>
      <p:identity/>
   </p:declare-step>
   <p:choose>
      <p:when test="p:step-available('ex:step2')">
         <p:identity>
            <p:with-input>
               <wrong/>
            </p:with-input>
         </p:identity>
      </p:when>
      <p:otherwise>
         <p:identity>
            <p:with-input>
               <right/>
            </p:with-input>
         </p:identity>
      </p:otherwise>
   </p:choose>
</p:declare-step>