@Override
    public V waitForValue() {
      return get();
    }
  }

  /** References a strong value. */
  private static class StrongValueReference<K, V> implements ValueReference<K, V> {
    final V referent;

    StrongValueReference(V referent) {
      this.referent = referent;
    }

    @Override
    public V get() {
      return referent;
    }

    @Override