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)));
    }
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
querydsl-core/src/main/java/com/querydsl/core/group/GroupBy.java