Initial review

The most recent commit message is so accurate.

I don't think this `locals()` cleverness is worth it to save just a couple of lines.  You could try something like:

```
for (typ, lst) in (("adds", adds), ("subs", subs)):
    for chunk in self.chunks[typ]:
        lst.append(chunk)
```
But it's probably clearer and simpler to just spell out each iteration independently.