The pipeline
<p:declare-step xmlns:ex="http://some-extension.org" xmlns:p="http://www.w3.org/ns/xproc" name="main" version="3.0">
<p:output port="result"/>
<p:declare-step type="ex:test" use-when="false()">
<p:identity>
<p:with-input>
<dummy/>
</p:with-input>
</p:identity>
</p:declare-step>
<p:declare-step type="ex:test1" use-when="p:step-available('ex:test')">
<p:identity>
<p:with-input>
<another-dummy/>
</p:with-input>
</p:identity>
</p:declare-step>
<p:identity>
<p:with-input>
<result>{p:step-available('ex:test1')}</result>
</p:with-input>
</p:identity>
</p:declare-step>
Schematron validation
<s:schema xmlns="http://www.w3.org/1999/xhtml" xmlns:s="http://purl.oclc.org/dsdl/schematron" queryBinding="xslt2">
<s:pattern>
<s:rule context="/">
<s:assert test="result">The document root is not “result”.</s:assert>
<s:assert test="result/text()='false'">Step is visible.</s:assert>
</s:rule>
</s:pattern>
</s:schema>
