如何使用mgo构建$ setIsSubset的聚合

I tried to construct a pipeline with mgo, but I have no idea about how to use $setIsSubset operator.

Here is the example from mongodb doc shows the usage of $setIsSubSet. Is there anyone who can give me some tips? :-)

<code>

db.experiments.aggregate(
   [
       {$project: {A:1, B:1,AisSubset: {$setIsSubset: ["$A", "$B"]}, _id:0 }}
   ]
)

</code>