code: 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));
      }
docs: 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