}

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

  static final class SynchronizedBiMap<K extends @Nullable Object, V extends @Nullable Object>