Multiple places in the $scoreFusion and $rankFusion implementation we iterate over a list of document source to append them to a list of other document sources. Like:

for (auto&& stage : someStages) {
  otherStages.emplace_back(stage);
} 

• here is an existing standard library function std::list::splice
• that does this type of operation succinctly in a single call. The example above can be written like:

otherStages.splice(otherStages.begin(), someStages);

This function generalizes to be able to insert stages to the back of another list, or in the middle.

Here are some places I've found were were doing this:

• here
• here
• here - this one is a std::vector (which doesn't have splice) as well as adding fields from a BSONObj so the most efficient implementation is probably the one written
• here - ditto
• here
• here

Its possible that by the time this ticket is addressed some of these example may not apply, or that there are other examples I did not identify. Regardless, when doing this ticket, the final state is that all of the for loop style stitchings are replaced with splice.