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

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

  @Override
  public boolean isEmpty() {
    return delegate.isEmpty();
  }

  @Override
  public Iterator<E> iterator() {
    return delegate.iterator();

  }

  @CollectionSize.Require(SEVERAL)
  public void testHigherHole() {
    resetWithHole();
    assertEquals(c, navigableSet.higher(a));
    assertEquals(c, navigableSet.higher(b));
    assertEquals(null, navigableSet.higher(c));
  }

  /*
   * TODO(cpovirk): make "too small" and "too large" elements available for better navigation

      @Override
      int resultIndex(int higherIndex) {
        return higherIndex - 1;
      }
    },
    /**
     * Return the index of the next higher element in the list, or {@code list.size()} if there is
     * no such element.
     */
    NEXT_HIGHER {
      @Override
      public int resultIndex(int higherIndex) {
        return higherIndex;
      }
    },
    /**

            KeyPresentBehavior.FIRST_AFTER,
            KeyAbsentBehavior.NEXT_HIGHER);
    int upperIndex =
        SortedLists.binarySearch(
            ranges,
            Range::lowerBound,
            range.upperBound,
            KeyPresentBehavior.ANY_PRESENT,
            KeyAbsentBehavior.NEXT_HIGHER);
    if (lowerIndex >= upperIndex) {
      return ImmutableRangeMap.of();
    }

  @CheckForNull
  public E lower(E element) {
    return forward.higher(element);
  }

  @Override
  @CheckForNull
  public E floor(E element) {
    return forward.ceiling(element);
  }

  @Override
  @CheckForNull
  public E ceiling(E element) {
    return forward.floor(element);
  }

  @Override
  @CheckForNull
  public E higher(E element) {
    return forward.lower(element);
  }

  }

  @CollectionSize.Require(SEVERAL)
  public void testHigherHole() {
    resetWithHole();
    assertEquals(c, navigableSet.higher(a));
    assertEquals(c, navigableSet.higher(b));
    assertEquals(null, navigableSet.higher(c));
  }

  /*
   * TODO(cpovirk): make "too small" and "too large" elements available for better navigation

              KeyAbsentBehavior.NEXT_HIGHER);
    } else {
      fromIndex = 0;
    }

    int toIndex;
    if (range.hasUpperBound()) {
      toIndex =
          SortedLists.binarySearch(
              ranges,
              Range::lowerBound,
              range.upperBound,
              KeyPresentBehavior.FIRST_PRESENT,
              KeyAbsentBehavior.NEXT_HIGHER);
    } else {

              KeyAbsentBehavior.NEXT_HIGHER);
    } else {
      fromIndex = 0;
    }

    int toIndex;
    if (range.hasUpperBound()) {
      toIndex =
          SortedLists.binarySearch(
              ranges,
              Range::lowerBound,
              range.upperBound,
              KeyPresentBehavior.FIRST_PRESENT,
              KeyAbsentBehavior.NEXT_HIGHER);
    } else {

  @CheckForNull
  public E ceiling(E element) {
    int index = tailIndex(element, true);
    return (index == size()) ? null : elements.get(index);
  }

  @Override
  @CheckForNull
  public E higher(E element) {
    int index = tailIndex(element, false);
    return (index == size()) ? null : elements.get(index);
  }

  @Override
  ImmutableSortedSet<E> headSetImpl(E toElement, boolean inclusive) {

            KeyPresentBehavior.FIRST_AFTER,
            KeyAbsentBehavior.NEXT_HIGHER);
    int upperIndex =
        SortedLists.binarySearch(
            ranges,
            Range::lowerBound,
            range.upperBound,
            KeyPresentBehavior.ANY_PRESENT,
            KeyAbsentBehavior.NEXT_HIGHER);
    if (lowerIndex >= upperIndex) {
      return ImmutableRangeMap.of();
    }