code: 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"));
      }
docs: Returns true if the given value is a subject reference.
  
  @param v
  the value to check.
  
  @return true if the value is a subject reference, false if not.