Depends with an extension

<p:declare-step xml:id="b2-extn"
		xmlns:p="http://www.w3.org/ns/xproc"
                xmlns:cx="http://xmlcalabash.com/ns/extensions">
  <p:input port="parameters" kind="parameter"/>

  <p:xinclude>
    <p:input port="source">
      <p:document href="langspec.xml"/>
    </p:input>
  </p:xinclude>

  <p:validate-with-relax-ng>
    <p:input port="schema">
      <p:document href="../schema/dbspec.rng"/>
    </p:input>
  </p:validate-with-relax-ng>

  <p:xslt>
    <p:input port="stylesheet">
      <p:document href="../style/dbspec.xsl"/>
    </p:input>
  </p:xslt>

  <p:store name="store" href="langspec.html"/>

  <p:exec result-is-xml="false"
          source-is-xml="false"
          command="/Users/ndw/bin/tidy"
          args="langspec.html"
          cx:depends-on="store">
    <p:input port="source"><p:empty/></p:input>
  </p:exec>

  <p:sink/>
</p:declare-step>