* consist strictly of loading values, though during refresh a value may be both active and
     * loading.
     */
    boolean isActive();
  }

  /** Placeholder. Indicates that the value hasn't been set yet. */
  static final ValueReference<Object, Object> UNSET =
      new ValueReference<Object, Object>() {
        @CheckForNull
        @Override
        public Object get() {

     * consist strictly of loading values, though during refresh a value may be both active and
     * loading.
     */
    boolean isActive();
  }

  /** Placeholder. Indicates that the value hasn't been set yet. */
  static final ValueReference<Object, Object> UNSET =
      new ValueReference<Object, Object>() {
        @CheckForNull
        @Override
        public Object get() {