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>