mongo operators

$elemMatch
https://docs.mongodb.com/manual/reference/operator/query/elemMatch/#op._S_elemMatch
$slice
https://docs.mongodb.com/manual/reference/operator/aggregation/slice/index.html
$filter
https://docs.mongodb.com/manual/reference/operator/aggregation/filter/index.html
$reduce
https://docs.mongodb.com/manual/reference/operator/aggregation/reduce/index.html
$arrayElemAt
https://docs.mongodb.com/manual/reference/operator/aggregation/arrayElemAt/index.html

$reverseArray
https://docs.mongodb.com/manual/reference/operator/aggregation/reverseArray/index.html

$type
https://docs.mongodb.com/manual/reference/operator/aggregation/type/index.html

$all
https://docs.mongodb.com/manual/reference/operator/query/all/index.html