code: public static <K extends Comparable<? super K>, V, U> AbstractGroupExpression<Pair<K, V>, SortedMap<K, U>> sortedMap(Expression<K> key,
                                                                                                                           GroupExpression<V, U> value) {
          return sortedMap(new GOne<K>(key), value);
      }
docs: Create a new aggregating map expression using a backing TreeMap
  
  @param key key for the map entries
  @param value value for the map entries
  @return wrapper expression