code: public static <E> AbstractGroupExpression<E, SortedSet<E>> sortedSet(Expression<E> expression,
                                                                           Comparator<? super E> comparator) {
          return GSet.createSorted(expression, comparator);
      }
docs: Create a new aggregating set expression using a backing TreeSet using the given comparator
  
  @param expression values for this expression will be accumulated into a set
  @param comparator comparator of the created TreeSet instance
  @return wrapper expression