The pipeline
<p:declare-step xmlns:p="http://www.w3.org/ns/xproc" version="3.0">
<p:output port="result"/>
<p:set-properties properties="map{'key':'new-value'}" merge="true">
<p:with-input>
<p:inline content-type="application/json" document-properties="map{'key':'value'}">"text"</p:inline>
</p:with-input>
</p:set-properties>
<p:cast-content-type content-type="application/xml">
<p:with-input select="p:document-properties(.)"/>
</p:cast-content-type>
</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:ns uri="http://www.w3.org/2005/xpath-functions" prefix="j"/>
<s:pattern>
<s:rule context="/">
<s:assert test="j:map">Root element is not 'j:map'.</s:assert>
<s:assert test="j:map/j:string[@key='content-type']">No 'content-type' property found.</s:assert>
<s:assert test="j:map/j:string[@key='content-type']/text()='application/json'">Element 'content-type' is not 'application/json'.</s:assert>
<s:assert test="j:map/j:string[@key='key']/text()='new-value'">No 'key' property with value 'new-value' found.</s:assert>
<s:assert test="j:map/j:string[@key='base-uri']">No 'base-uri' property found.</s:assert>
<s:assert test="count(j:map/*)=3">Root element does not have exactly two children.</s:assert>
</s:rule>
</s:pattern>
</s:schema>
