@GwtIncompatible // serialization
  @J2ktIncompatible
  @SuppressWarnings("unused")
  private static class KeySetSerializedForm<K> implements Serializable {
    final ImmutableMap<K, ?> map;

    KeySetSerializedForm(ImmutableMap<K, ?> map) {
      this.map = map;
    }

    Object readResolve() {
      return map.keySet();
    }