$accumulator(AGGREGATION_EXPRESSION)
$addToSet(AGGREGATION_EXPRESSION)
$avg(AGGREGATION_EXPRESSION)
$count(AGGREGATION_EXPRESSION)
$first(AGGREGATION_EXPRESSION)
$last(AGGREGATION_EXPRESSION)
$max(AGGREGATION_EXPRESSION)
$mergeObjects(AGGREGATION_EXPRESSION)
$min(AGGREGATION_EXPRESSION)
$push(AGGREGATION_EXPRESSION)
$stdDevPop(AGGREGATION_EXPRESSION)
$stdDevSamp(AGGREGATION_EXPRESSION)
$sum(AGGREGATION_EXPRESSION)