* additional values interleaved with those shown above.)
     */
    @CheckForNull
    abstract N visitNext(Deque<Iterator<? extends N>> horizon);
  }

  /** Poor man's method reference for {@code Deque::addFirst} and {@code Deque::addLast}. */
  private enum InsertionOrder {
    FRONT {
      @Override
      <T> void insertInto(Deque<T> deque, T value) {
        deque.addFirst(value);

all came different!' Alice replied in a very melancholy voice.

  `Repeat, "YOU ARE OLD, FATHER WILLIAM,"' said the Caterpillar.

  Alice folded her hands, and began:--

    `You are old, Father William,' the young man said,
      `And your hair has become very white;
    And yet you incessantly stand on your head--
      Do you think, at your age, it is right?'

    `In my youth,' Father William replied to his son,

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all came different!' Alice replied in a very melancholy voice.

  `Repeat, "YOU ARE OLD, FATHER WILLIAM,"' said the Caterpillar.

  Alice folded her hands, and began:--

    `You are old, Father William,' the young man said,
      `And your hair has become very white;
    And yet you incessantly stand on your head--
      Do you think, at your age, it is right?'

    `In my youth,' Father William replied to his son,