The pipeline
<p:declare-step xmlns:imp="http://test" xmlns:p="http://www.w3.org/ns/xproc" name="main" version="3.0">
<p:output port="result"/>
<p:declare-step type="imp:a" use-when="not(p:step-available('imp:b'))">
<p:output port="result"/>
<p:identity>
<p:with-input>
<doc-A/>
</p:with-input>
</p:identity>
</p:declare-step>
<p:declare-step type="imp:b" use-when="true()">
<p:output port="result"/>
<p:identity>
<p:with-input>
<doc-B/>
</p:with-input>
</p:identity>
</p:declare-step>
<p:identity>
<p:with-input>
<result>
<test>{p:step-available('imp:a')}</test>
</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/test/text()='false'">p:step-available for 'test:step1' is wrong.</s:assert>
</s:rule>
</s:pattern>
</s:schema>
