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}.
   *