code: public static <K, V, T extends Comparable<? super T>, U> AbstractGroupExpression<Pair<K, V>, SortedMap<T, U>> sortedMap(GroupExpression<K, T> key,
                                                                                                                              GroupExpression<V, U> value) {
          return new GMap.Mixin<K, V, T, U, SortedMap<T, U>>(key, value, GMap.createSorted(QPair.create(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