@SuppressWarnings("unchecked")
  static <K extends @Nullable Object> Function<Entry<K, ?>, K> keyFunction() {
    return (Function) EntryFunction.KEY;
  }

  @SuppressWarnings("unchecked")
  static <V extends @Nullable Object> Function<Entry<?, V>, V> valueFunction() {
    return (Function) EntryFunction.VALUE;
  }

  static <K extends @Nullable Object, V extends @Nullable Object> Iterator<K> keyIterator(