docs: protected static Document replaceSystemPropertyVariables( Document doc ) {
          if (doc.isEmpty()) return doc;
          Document modified = doc.withVariablesReplacedWithSystemProperties();
          if (modified == doc) return doc;
  
          // Otherwise, we changed some values. Note that the system properties can only be used in
          // string values, whereas the schema may expect non-string values. Therefore, we need to validate
          // the document against the schema and possibly perform some conversions of values ...
          return SCHEMA_LIBRARY.convertValues(modified, JSON_SCHEMA_URI);
      }