@J2ktIncompatible // Synchronized
  public void testSynchronizedBiMap() {
    BiMap<String, Integer> bimap = HashBiMap.create();
    bimap.put("one", 1);
    BiMap<String, Integer> sync = Maps.synchronizedBiMap(bimap);
    bimap.put("two", 2);
    sync.put("three", 3);
    assertEquals(ImmutableSet.of(1, 2, 3), bimap.inverse().keySet());
    assertEquals(ImmutableSet.of(1, 2, 3), sync.inverse().keySet());
  }

  }

  static <K extends @Nullable Object, V extends @Nullable Object> BiMap<K, V> biMap(
      BiMap<K, V> bimap, @Nullable Object mutex) {
    if (bimap instanceof SynchronizedBiMap || bimap instanceof ImmutableBiMap) {
      return bimap;
    }
    return new SynchronizedBiMap<>(bimap, mutex, null);
  }

  }

  static <K extends @Nullable Object, V extends @Nullable Object> BiMap<K, V> biMap(
      BiMap<K, V> bimap, @Nullable Object mutex) {
    if (bimap instanceof SynchronizedBiMap || bimap instanceof ImmutableBiMap) {
      return bimap;
    }
    return new SynchronizedBiMap<>(bimap, mutex, null);
  }