public static <E extends Comparable<? super E>> AbstractGroupExpression<E, SortedSet<E>> sortedSet(Expression<E> expression) {
        return GSet.createSorted(expression);
    }
Create a new aggregating set expression using a backing TreeSet

@param expression values for this expression will be accumulated into a set
@return wrapper expression
querydsl-core/src/main/java/com/querydsl/core/group/GroupBy.java