code: public static <K, V, T> AbstractGroupExpression<Pair<K, V>, Map<T, V>> map(GroupExpression<K, T> key,
                                                                                 Expression<V> value) {
          return map(key, new GOne<V>(value));
      }
docs: Create a new aggregating map expression using a backing LinkedHashMap
  
  @param key key for the map entries
  @param value value for the map entries
  @return wrapper expression