presto-main/src/main/java/io/prestosql/operator/StreamingAggregationOperator.java/StreamingAggregationOperator/setupAggregates
class StreamingAggregationOperator:    private List<Aggregator> setupAggregates(Step step, List<AccumulatorFactory> accumulatorFactories)
    {
        ImmutableList.Builder<Aggregator> builder = ImmutableList.builder();
        for (AccumulatorFactory factory : accumulatorFactories) {
            builder.add(new Aggregator(factory, step));
        }
        return builder.build();
    }