protected function convertToScalarImpl(Aggregator $aggregator)
 {
     /* @var $aggregator GroupingAggregator */
     return array_map(function (AggregatorOfSubjectsWithCommonProperties $subAggregator) {
         return $this->presenterOfActualAggregators->convertToScalar($subAggregator);
     }, $aggregator->getAggregatorsOfSubjectsWithCommonProperties());
 }