@SuppressWarnings("unchecked")
  private static <K extends @Nullable Object, V extends @Nullable Object>
      Function<Map<K, V>, Map<K, V>> unmodifiableWrapper() {
    return (Function) UNMODIFIABLE_WRAPPER;
  }

  private static final Function<? extends Map<?, ?>, ? extends Map<?, ?>> UNMODIFIABLE_WRAPPER =
      new Function<Map<Object, Object>, Map<Object, Object>>() {
        @Override

  @SuppressWarnings("unchecked")
  private static <K extends @Nullable Object, V extends @Nullable Object>
      Function<Map<K, V>, Map<K, V>> unmodifiableWrapper() {
    return (Function) UNMODIFIABLE_WRAPPER;
  }

  private static final Function<? extends Map<?, ?>, ? extends Map<?, ?>> UNMODIFIABLE_WRAPPER =
      new Function<Map<Object, Object>, Map<Object, Object>>() {
        @Override