* nulls as long as the *last* element is *not* null.
   *
   * (To "go out of its way," the method tracks a `set` bit so that it can distinguish "the final
   * split has a last element of null, so throw NPE" from "the final split was empty, so look for an
   * element in the prior one.")
   */
  public static <T> java.util.Optional<T> findLast(Stream<T> stream) {
    class OptionalState {
      boolean set = false;

    if (occurrences == 0) {
      return count(element);
    }
    checkArgument(occurrences > 0, "occurrences cannot be negative: %s", occurrences);
    int entryIndex = backingMap.indexOf(element);
    if (entryIndex == -1) {
      backingMap.put(element, occurrences);
      size += occurrences;
      return 0;
    }
    int oldCount = backingMap.getValue(entryIndex);

    @Override
    protected List<Short> create(Short[] elements) {
      return asList(elements);
    }
  }

  public static final class ShortsAsListHeadSubListGenerator extends TestShortListGenerator {
    @Override
    protected List<Short> create(Short[] elements) {
      Short[] suffix = {Short.MIN_VALUE, Short.MAX_VALUE};
      Short[] all = concat(elements, suffix);
      return asList(all).subList(0, elements.length);

    protected List<Character> create(Character[] elements) {
      return asList(elements);
    }
  }

  public static final class CharsAsListHeadSubListGenerator extends TestCharListGenerator {
    @Override
    protected List<Character> create(Character[] elements) {
      Character[] suffix = {Character.MIN_VALUE, Character.MAX_VALUE};
      Character[] all = concat(elements, suffix);

    @Override
    protected List<Short> create(Short[] elements) {
      return asList(elements);
    }
  }

  public static final class ShortsAsListHeadSubListGenerator extends TestShortListGenerator {
    @Override
    protected List<Short> create(Short[] elements) {
      Short[] suffix = {Short.MIN_VALUE, Short.MAX_VALUE};
      Short[] all = concat(elements, suffix);
      return asList(all).subList(0, elements.length);

    protected List<Long> create(Long[] elements) {
      return asList(elements);
    }
  }

  public static final class LongsAsListHeadSubListGenerator extends TestLongListGenerator {
    @Override
    protected List<Long> create(Long[] elements) {
      Long[] suffix = {Long.MIN_VALUE, Long.MAX_VALUE};
      Long[] all = concat(elements, suffix);
      return asList(all).subList(0, elements.length);
    }
  }

  /** Constructor for use by subclasses. */
  protected AbstractIterator() {}

  private enum State {
    /** We have computed the next element and haven't returned it yet. */
    READY,

    /** We haven't yet computed or have already returned the element. */
    NOT_READY,

    /** We have reached the end of the data and are finished. */
    DONE,

    /** We've suffered an exception and are kaput. */

    @Override
    public int count(@Nullable Object element) {
      return standardCount(element);
    }

    @Override
    public boolean equals(@Nullable Object object) {
      return standardEquals(object);
    }

    @Override
    public int hashCode() {
      return standardHashCode();
    }

    @Override
    public boolean add(E element) {
      return standardAdd(element);
    }

    @Override
    public int count(@Nullable Object element) {
      return standardCount(element);
    }

    @Override
    public boolean equals(@Nullable Object object) {
      return standardEquals(object);
    }

    @Override
    public int hashCode() {
      return standardHashCode();
    }

    @Override
    public boolean add(E element) {
      return standardAdd(element);
    }

              @Override
              public Collection<@Nullable String> create(@Nullable String[] elements) {
                List<@Nullable String> list = newArrayList();
                for (String element : elements) {
                  list.add((element == null) ? null : "q" + element);
                }
                return Collections2.transform(