*/
@GwtCompatible
abstract class AbstractBiMap<K extends @Nullable Object, V extends @Nullable Object>
    extends ForwardingMap<K, V> implements BiMap<K, V>, Serializable {

  private transient Map<K, V> delegate;

  @RetainedWith private transient AbstractBiMap<V, K> inverse;

  /** Package-private constructor for creating a map-backed bimap. */
  AbstractBiMap(Map<K, V> forward, Map<V, K> backward) {

 */
@GwtCompatible
abstract class AbstractBiMap<K extends @Nullable Object, V extends @Nullable Object>
    extends ForwardingMap<K, V> implements BiMap<K, V>, Serializable {

  private transient Map<K, V> delegate;

  @RetainedWith private transient AbstractBiMap<V, K> inverse;

  /** Package-private constructor for creating a map-backed bimap. */
  AbstractBiMap(Map<K, V> forward, Map<V, K> backward) {

   * as implementation details.
   *
   * <p>Since the bimap is immutable, ImmutableBiMap doesn't require special logic for keeping the
   * bimap and its inverse in sync during serialization, the way AbstractBiMap does.
   */
  @J2ktIncompatible // serialization
  private static final class SerializedForm<K, V> extends ImmutableMap.SerializedForm<K, V> {
    SerializedForm(ImmutableBiMap<K, V> bimap) {
      super(bimap);