Step available 003 (NW)

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:identity use-when="p:step-available('ex:step2')"/>
   </p:declare-step>
   <p:declare-step type="ex:step2">
      <p:input port="source"/>
      <p:output port="result"/>
   </p:declare-step>
   <p:identity>
      <p:with-input use-when="p:step-available('ex:step1')">
         <available/>
      </p:with-input>
      <p:with-input use-when="not(p:step-available('ex:step1'))">
         <not-available/>
      </p:with-input>
   </p:identity>
</p:declare-step>