public static <E> AbstractGroupExpression<E, SortedSet<E>> sortedSet(Expression<E> expression,
                                                                         Comparator<? super E> comparator) {
        return GSet.createSorted(expression, comparator);
    }
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
querydsl-core/src/main/java/com/querydsl/core/group/GroupBy.java