*/
  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 final 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 final class UnmodifiableBiMap<
          K extends @Nullable Object, V extends @Nullable Object>

    BiMap<Integer, String> mod = HashBiMap.create();
    mod.put(1, "one");
    mod.put(2, "two");
    mod.put(3, "three");

    BiMap<Number, String> unmod = Maps.<Number, String>unmodifiableBiMap(mod);

    /* No aliasing on inverse operations. */
    assertThat(unmod.inverse()).isSameInstanceAs(unmod.inverse());
    assertThat(unmod.inverse().inverse()).isSameInstanceAs(unmod);