public static <E, F> GroupExpression<E, SortedSet<F>> sortedSet(GroupExpression<E, F> groupExpression,
                                                                    Comparator<? super F> comparator) {
        return new MixinGroupExpression<E, F, SortedSet<F>>(groupExpression, GSet.createSorted(groupExpression, comparator));
    }
Create a new aggregating set expression using a backing TreeSet using the given comparator

@param groupExpression 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