public @Nullable E floor(E e) {
    return delegate.floor(checkValid(e));
  }

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

  @Override
  public NavigableSet<E> headSet(E toElement, boolean inclusive) {
    return new SafeTreeSet<>(delegate.headSet(checkValid(toElement), inclusive));
  }

  @Override
  public @Nullable E higher(E e) {

  public @Nullable E floor(E e) {
    return delegate.floor(checkValid(e));
  }

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

  @Override
  public NavigableSet<E> headSet(E toElement, boolean inclusive) {
    return new SafeTreeSet<>(delegate.headSet(checkValid(toElement), inclusive));
  }

  @Override
  public @Nullable E higher(E e) {

    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();
    assertThat(set.headSet("a")).isSameInstanceAs(this.<String>of());
    assertThat(set.headSet("b")).isSameInstanceAs(this.<String>of());
  }

    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();
    assertThat(set.headSet("a")).isSameInstanceAs(this.<String>of());
    assertThat(set.headSet("b")).isSameInstanceAs(this.<String>of());
  }

      }

      @Override
      public SortedSet<E> headSet(@ParametricNullness E toElement) {
        return removeOnlySortedSet(super.headSet(toElement));
      }

      @Override
      public NavigableSet<E> headSet(@ParametricNullness E toElement, boolean inclusive) {
        return removeOnlyNavigableSet(super.headSet(toElement, inclusive));
      }

      @Override

      }

      @Override
      public SortedSet<E> headSet(@ParametricNullness E toElement) {
        return removeOnlySortedSet(super.headSet(toElement));
      }

      @Override
      public NavigableSet<E> headSet(@ParametricNullness E toElement, boolean inclusive) {
        return removeOnlyNavigableSet(super.headSet(toElement, inclusive));
      }

      @Override

  }

  public void testRowKeySetHeadSet() {
    sortedTable = create("foo", 1, 'a', "bar", 1, 'b', "foo", 3, 'c');
    Set<String> set = sortedTable.rowKeySet().headSet("cat");
    assertEquals(singleton("bar"), set);
    set.clear();
    assertTrue(set.isEmpty());
    assertEquals(singleton("foo"), sortedTable.rowKeySet());
  }

  public void testRowKeySetTailSet() {

    assertThat(keySet.first()).isNull();
    assertThat(keySet.last()).isEqualTo("google");
    assertEquals(StringLength.COMPARATOR, keySet.comparator());
    assertEquals(Sets.<@Nullable String>newHashSet(null, "tree"), keySet.headSet("yahoo"));
    assertEquals(newHashSet("google"), keySet.tailSet("yahoo"));
    assertEquals(newHashSet("tree"), keySet.subSet("ask", "yahoo"));
  }

  @GwtIncompatible // SerializableTester

    assertThat(elementSet.first()).isEqualTo("a");
    assertThat(elementSet.last()).isEqualTo("c");
    assertEquals(Ordering.natural(), elementSet.comparator());

    assertThat(elementSet.headSet("b")).containsExactly("a");
    assertThat(elementSet.tailSet("b")).containsExactly("b", "c").inOrder();
    assertThat(elementSet.subSet("a", "c")).containsExactly("a", "b").inOrder();
  }

      Header("content-disposition", ""),
      Header("content-encoding", ""),
      Header("content-language", ""),
      Header("content-length", ""),
      Header("content-location", ""),
      Header("content-range", ""),
      Header("content-type", ""),
      Header("cookie", ""),
      Header("date", ""),
      Header("etag", ""),
      Header("expect", ""),
      Header("expires", ""),