It requires the following features: xquery-function-import or xslt-function-import.
The pipeline
<p:declare-step xmlns:err="http://www.w3.org/ns/xproc-error" xmlns:p="http://www.w3.org/ns/xproc" xmlns:test="http://xproc.org/ns/testsuite/3.0/function-test" version="3.0">
<p:import-functions href="../documents/xquery-library3.xq" content-type="application/xquery"/>
<p:import-functions href="../documents/xslt-functions3.xsl" content-type="application/xslt+xml"/>
<p:output port="result"/>
<p:identity>
<p:with-input>
<result>{test:function1('arg')}</result>
</p:with-input>
</p:identity>
</p:declare-step>
