int index = tailIndex(element, false);
    return (index == size()) ? null : elements.get(index);
  }

  @Override
  ImmutableSortedSet<E> headSetImpl(E toElement, boolean inclusive) {
    return getSubSet(0, headIndex(toElement, inclusive));
  }

  int headIndex(E toElement, boolean inclusive) {
    int index = Collections.binarySearch(elements, checkNotNull(toElement), comparator());
    if (index >= 0) {

    if (fromIndex == 0 && toIndex == size()) {
      return this;
    } else if (fromIndex == toIndex) {
      return emptyMap(comparator());
    } else {
      return new ImmutableSortedMap<>(
          keySet.getSubSet(fromIndex, toIndex), valueList.subList(fromIndex, toIndex));
    }
  }

  /**
   * This method returns a {@code ImmutableSortedMap}, consisting of the entries whose keys are less
   * than {@code toKey}.
   *