Currently set operations require several steps of pipeline manipulation to produce the desired result. For example, see StackOverflow: How to use MongoDB aggregation for general purpose set operations (union, intersection, difference).
This is a feature request to add some new operations for more efficient processing in a $project pipeline stage:
1) $seq - similar to $eq, takes two sets and returns true if equal, false if not. If two lists are passed in, treat as multi-set
2) $ssub - is the first set a subet of the second, returns true if so
3) $sint - takes in two sets and provides the intersection of the two
4) $sdiff - provides the symetric difference between two sets
5) $sunion - provides the union of two sets