@Override
        public Object waitForValue() {
          return null;
        }

        @Override
        public void notifyNewValue(Object newValue) {}
      };

  /** Singleton placeholder that indicates a value is being loaded. */
  @SuppressWarnings("unchecked") // impl never uses a parameter or returns any non-null value
  static <K, V> ValueReference<K, V> unset() {

I suppose?'

  `Yes,' said Alice doubtfully:  `it means--to--make--anything--prettier.'

  `Well, then,' the Gryphon went on, `if you don't know what to
uglify is, you ARE a simpleton.'

  Alice did not feel encouraged to ask any more questions about
it, so she turned to the Mock Turtle, and said `What else had you
to learn?'

  `Well, there was Mystery,' the Mock Turtle replied, counting