what is the value of an "update or insert" call on `Map`? is that not just set?

`getOrInsert` here seems to be the Python "set_default" method on dicts, which is very useful at avoiding tedium in some basic data munging

The proposal used to do more thing and we didn't change the URL after we ultimately arrived at this set of APIs.

Reminds me of the weird C++ map operator[] behavior.

If you use that operator and the value doesn't exist, it'll default create one and return a reference to that.

And as I'm writing this I realize why... references cannot be `null`.