*/
    final Stack<E> previousElements = new Stack<>();

    /**
     * {@link #nextElements} if {@code next()} was called more recently then {@code previous},
     * {@link #previousElements} if the reverse is true, or -- overriding both of these -- {@code
     * null} if {@code remove()} or {@code add()} has been called more recently than either. We use

  @SuppressWarnings("CatchingUnchecked") // sneaky checked exception
  public final void test() {
    try {
      recurse(0);
    } catch (Exception e) { // sneaky checked exception
      throw new RuntimeException(Arrays.toString(stimuli), e);
    }
  }

  private void recurse(int level) {
    // We're going to reuse the stimuli array 3^steps times by overwriting it
    // in a recursive loop.  Sneaky.