@Override
        public @Nullable 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() {

  public static <T> Ordering<T> explicit(T leastValue, T... remainingValuesInOrder) {
    return explicit(Lists.asList(leastValue, remainingValuesInOrder));
  }

  // Ordering<Object> singletons

  /**
   * Returns an ordering which treats all values as equal, indicating "no ordering." Passing this
   * ordering to any <i>stable</i> sort algorithm results in no change to the order of elements.

  public static <T> Ordering<T> explicit(T leastValue, T... remainingValuesInOrder) {
    return explicit(Lists.asList(leastValue, remainingValuesInOrder));
  }

  // Ordering<Object> singletons

  /**
   * Returns an ordering which treats all values as equal, indicating "no ordering." Passing this
   * ordering to any <i>stable</i> sort algorithm results in no change to the order of elements.