* circular dependency is present, so the proxy must be able to behave as the map itself.
   */
  private static final class SerializationProxy<K, V> extends AbstractSerializationProxy<K, V> {
    private static final long serialVersionUID = 3;

    SerializationProxy(
        Strength keyStrength,
        Strength valueStrength,
        Equivalence<Object> keyEquivalence,