class AggregatorPipelineExtractor {

  public Map<Aggregator<?, ?>, Collection<PTransform<?, ?>>> getAggregatorSteps() {
    HashMultimap<Aggregator<?, ?>, PTransform<?, ?>> aggregatorSteps = HashMultimap.create();
    pipeline.traverseTopologically(new AggregatorVisitor(aggregatorSteps));
    return aggregatorSteps.asMap();
  }

  public  AggregatorPipelineExtractor(Pipeline pipeline);

}