code: public static <E, F> GroupExpression<E, Set<F>> set(GroupExpression<E, F> groupExpression) {
          return new MixinGroupExpression<E, F, Set<F>>(groupExpression, GSet.createLinked(groupExpression));
      }
docs: Create a new aggregating set expression using a backing LinkedHashSet
  
  @param groupExpression values for this expression will be accumulated into a set
  @param <E>
  @param <F>
  @return wrapper expression