This ticket tracks the work to make a helper function to perform rank fusion. Something like the following, but tested out:

auto singleRRFTerm(const std::vector<Value>& ks) {
using D = Document;
return D {
{
"$divide"_sd, std::vector {
1, D {
{
"$add"_sd, std::vector<ImplicitValue> {
"$rank", D {
{
"$arrayElemAt"_sd, std::vector

}
}
}
}
}
}
}
}
}