public function &getAggregatorsApplyingOn(ExportInterface $export, array $centers = null)
    {
        foreach ($this->aggregators as $alias => $aggregator) {
            if (in_array($aggregator->applyOn(), $export->supportsModifiers()) && 
                    $this->isGrantedForElement($aggregator, $export, $centers)) {
                yield $alias => $aggregator;
            }
        }
    }