code: public static <K, V, T, U> AbstractGroupExpression<Pair<K, V>, Map<T, U>> map(GroupExpression<K, T> key,
                                                                                    GroupExpression<V, U> value) {
          return new GMap.Mixin<K, V, T, U, Map<T, U>>(key, value, GMap.createLinked(QPair.create(key, 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