@GwtIncompatible
    @J2ktIncompatible
        private void writeObject(ObjectOutputStream stream) throws IOException {
      stream.defaultWriteObject();
      stream.writeObject(factory);
      stream.writeObject(backingMap());
    }

    @GwtIncompatible
    @J2ktIncompatible
        @SuppressWarnings("unchecked") // reading data stored by writeObject

   * @return an empty collection of values
   */
  Collection<V> createCollection(@ParametricNullness K key) {
    return createCollection();
  }

  Map<K, Collection<V>> backingMap() {
    return map;
  }

  // Query Operations

  @Override
  public int size() {
    return totalSize;
  }

  @Override
  public boolean containsKey(@Nullable Object key) {