* the original {@code toKey}.
   *
   * @since 12.0
   */
  @Override
  public ImmutableSortedMap<K, V> headMap(K toKey, boolean inclusive) {
    return getSubMap(0, keySet.headIndex(checkNotNull(toKey), inclusive));
  }

  /**
   * This method returns a {@code ImmutableSortedMap}, consisting of the entries whose keys ranges
   * from {@code fromKey}, inclusive, to {@code toKey}, exclusive.