* This method returns a {@code ImmutableSortedMap}, consisting of the entries whose keys ranges
   * from {@code fromKey}, inclusive, to {@code toKey}, exclusive.
   *
   * <p>The {@link SortedMap#subMap} documentation states that a submap of a submap throws an {@link
   * IllegalArgumentException} if passed a {@code fromKey} less than an earlier {@code fromKey}.
   * However, this method doesn't throw an exception in that situation, but instead keeps the

   * This method returns a {@code ImmutableSortedMap}, consisting of the entries whose keys ranges
   * from {@code fromKey}, inclusive, to {@code toKey}, exclusive.
   *
   * <p>The {@link SortedMap#subMap} documentation states that a submap of a submap throws an {@link
   * IllegalArgumentException} if passed a {@code fromKey} less than an earlier {@code fromKey}.
   * However, this method doesn't throw an exception in that situation, but instead keeps the

    }
    return headMap(toKey);
  }

  public ImmutableSortedMap<K, V> subMap(K fromKey, K toKey) {
    checkNotNull(fromKey);
    checkNotNull(toKey);
    checkArgument(comparator.compare(fromKey, toKey) <= 0);
    return newView(sortedDelegate.subMap(fromKey, toKey));
  }

  ImmutableSortedMap<K, V> subMap(K fromKey, boolean fromInclusive, K toKey, boolean toInclusive) {
    checkNotNull(fromKey);