code: public final ImmutableSortedSet<E> toSortedSet(Comparator<? super E> comparator) {
      return ImmutableSortedSet.copyOf(comparator, iterable);
    }
docs: Returns an {@code ImmutableSortedSet} containing all of the elements from this {@code
  FluentIterable} in the order specified by {@code comparator}, with duplicates (determined by
  {@code comparator.compare(x, y) == 0}) removed. To produce an {@code ImmutableSortedSet} sorted
  by its natural ordering, use {@code toSortedSet(Ordering.natural())}.
  
  @param comparator the function by which to sort set elements
  @throws NullPointerException if any element is null
  @since 14.0 (since 12.0 as {@code toImmutableSortedSet()}).