SortedSet<String> set = of("e", "f", "b", "d", "c");
    assertTrue(set.headSet("e") instanceof ImmutableSortedSet);
    assertThat(set.headSet("e")).containsExactly("b", "c", "d").inOrder();
    assertThat(set.headSet("g")).containsExactly("b", "c", "d", "e", "f").inOrder();
    assertSame(this.<String>of(), set.headSet("a"));
    assertSame(this.<String>of(), set.headSet("b"));
  }

  public void testOf_tailSet() {

      for (List<Integer> contents : SAMPLE_INPUTS) {
        for (int i = 0; i < 10; i++) {
          assertEquals(
              filter((C) createUnfiltered(contents).headSet(i), EVEN),
              filter(createUnfiltered(contents), EVEN).headSet(i));
        }
      }
    }

    @SuppressWarnings("unchecked")
    public void testTailSet() {
      for (List<Integer> contents : SAMPLE_INPUTS) {

   *
   * <p>The {@link SortedSet#headSet} documentation states that a subset of a subset throws an
   * {@link IllegalArgumentException} if passed a {@code toElement} greater than an earlier {@code
   * toElement}. However, this method doesn't throw an exception in that situation, but instead
   * keeps the original {@code toElement}.
   */
  @Override
  public ImmutableSortedSet<E> headSet(E toElement) {

    @Override
    protected SortedSet<String> create(String[] elements) {
      List<String> list = Lists.newArrayList(elements);
      list.add("zzz");
      return ImmutableSortedSet.copyOf(list).headSet("zzy");
    }
  }

  public static class ImmutableSortedSetTailsetGenerator extends TestStringSortedSetGenerator {
    @Override
    protected SortedSet<String> create(String[] elements) {

   *
   * <p>The {@link SortedSet#headSet} documentation states that a subset of a subset throws an
   * {@link IllegalArgumentException} if passed a {@code toElement} greater than an earlier {@code
   * toElement}. However, this method doesn't throw an exception in that situation, but instead
   * keeps the original {@code toElement}.
   */
  @Override
  public ImmutableSortedSet<E> headSet(E toElement) {

    NavigableSet<E> createSubSet(SortedSet<E> sortedSet, E firstExclusive, E lastExclusive) {
      NavigableSet<E> set = (NavigableSet<E>) sortedSet;
      if (from == Bound.NO_BOUND && to == Bound.INCLUSIVE) {
        return set.headSet(lastInclusive, true);
      } else if (from == Bound.EXCLUSIVE && to == Bound.NO_BOUND) {
        return set.tailSet(firstExclusive, false);
      } else if (from == Bound.EXCLUSIVE && to == Bound.EXCLUSIVE) {

    NavigableSet<E> createSubSet(SortedSet<E> sortedSet, E firstExclusive, E lastExclusive) {
      NavigableSet<E> set = (NavigableSet<E>) sortedSet;
      if (from == Bound.NO_BOUND && to == Bound.INCLUSIVE) {
        return set.headSet(lastInclusive, true);
      } else if (from == Bound.EXCLUSIVE && to == Bound.NO_BOUND) {
        return set.tailSet(firstExclusive, false);
      } else if (from == Bound.EXCLUSIVE && to == Bound.EXCLUSIVE) {

      }
    }

    @Override
    public NavigableSet<E> headSet(E toElement, boolean inclusive) {
      synchronized (mutex) {
        return Synchronized.navigableSet(delegate().headSet(toElement, inclusive), mutex);
      }
    }

    @Override
    public SortedSet<E> headSet(E toElement) {
      return headSet(toElement, false);
    }

    @Override
    @CheckForNull

      }
    }

    @Override
    public NavigableSet<E> headSet(E toElement, boolean inclusive) {
      synchronized (mutex) {
        return Synchronized.navigableSet(delegate().headSet(toElement, inclusive), mutex);
      }
    }

    @Override
    public SortedSet<E> headSet(E toElement) {
      return headSet(toElement, false);
    }

    @Override
    @CheckForNull

    @Override
    protected SortedSet<String> create(String[] elements) {
      List<String> list = Lists.newArrayList(elements);
      list.add("zzz");
      return ImmutableSortedSet.copyOf(list).headSet("zzy");
    }
  }

  public static class ImmutableSortedSetTailsetGenerator extends TestStringSortedSetGenerator {
    @Override
    protected SortedSet<String> create(String[] elements) {