presto-main/src/main/java/io/prestosql/operator/aggregation/multimapagg/MultimapAggregationFunction.java/MultimapAggregationFunction/combine
class MultimapAggregationFunction:    public static void combine(MultimapAggregationState state, MultimapAggregationState otherState)
    {
        state.merge(otherState);
    }