}
  }

  /** References a strong value. */
  private static final class WeightedStrongValueReference<K, V> extends StrongValueReference<K, V> {
    final int weight;

    WeightedStrongValueReference(V referent, int weight) {
      super(referent);
      this.weight = weight;
    }

    @Override
    public int getWeight() {

    }
  }

  /** References a strong value. */
  private static final class WeightedStrongValueReference<K, V> extends StrongValueReference<K, V> {
    final int weight;

    WeightedStrongValueReference(V referent, int weight) {
      super(referent);
      this.weight = weight;
    }

    @Override
    public int getWeight() {