@Override
        public String process(Document document, String output) {
          // make sure there are no left-overs from the split logic above
          // where the include processor inserts markers
          // that are then transformed in the tree processor
          if (output.contains(includeSourceMarker)) {
            throw new AssertionError("The include source linker left an include source marker somewhere");
          }
          return output;
        }
As we do custom include handling here, we have to reimplement all attribute handling we need.
So verify that only currently supported attributes are used.

@param reader The current reader
@param attributes The attributes used on the include directive
build-logic/asciidoc-extensions/src/main/java/org/spockframework/plugins/asciidoctor/IncludedSourceLinker.java