*/
  public static <K extends @Nullable Object, V extends @Nullable Object>
      BiMap<K, V> unmodifiableBiMap(BiMap<? extends K, ? extends V> bimap) {
    return new UnmodifiableBiMap<>(bimap, null);
  }

  /**
   * @see Maps#unmodifiableBiMap(BiMap)
   */
  private static class UnmodifiableBiMap<K extends @Nullable Object, V extends @Nullable Object>

   */
  public static <K extends @Nullable Object, V extends @Nullable Object>
      BiMap<K, V> unmodifiableBiMap(BiMap<? extends K, ? extends V> bimap) {
    return new UnmodifiableBiMap<>(bimap, null);
  }

  /**
   * @see Maps#unmodifiableBiMap(BiMap)
   */
  private static class UnmodifiableBiMap<K extends @Nullable Object, V extends @Nullable Object>