docs: static boolean isNodeReference(Object v) {
          // Note: A value is a subject reference if all of these hold true:
          // 1. It is an Object.
          // 2. It has a single key: @id.
          return (v instanceof Map && ((Map<String, Object>) v).size() == 1
                  && ((Map<String, Object>) v).containsKey("@id"));
      }