class AggregatingIndexSampler implements IndexSampler {

    @Override
    public IndexSample sampleIndex()
    {
        return indexSamplers.parallelStream()
                .map( this::sampleIndex )
                .reduce( this::combine )
                .get();
    }

    public  AggregatingIndexSampler( List<IndexSampler> indexSamplers );

}