Sean MacAvaney

Nah, I really see this as an opportunity to fix a ton of the rough edges in the current API. If we don't share this vision, perhaps this API should...

I sorta think kwargs add complications in this setting. E.g., what if competing values are provided? How about complementary values that need to be merged in properties/controls? Etc. I'm also...

We'd still need to make decisions on how they are handled. I'd rather selectively and carefully add functionality to `TerrierRetriever`. There's always the existing API for those who want more...

What functionality are you looking for, btw? I've added the bits that I use frequently.

IIRC that's still an open debate.

For (1) [`ABC`](https://docs.python.org/3/library/abc.html#abc.ABC) handles this somehow. Maybe we dig in a bit to see how it's done there? (2) isn't so common, I suppose it could just be up to...

> I think the API change may be backwards compatible I think you're right, and in the cases where it doesn't work, I'm happy with having it as a breaking...

> Question: can we somehow know how many times DataFrames are constructed, and check that this makes an indexing pipeline faster? Perhaps we could patch `pandas.DataFrame` with one that counts...

Alright, @cmacdonald can you take a look at my change? With another look, I'm not so jazzed that somebody who accidentally uses an `Indexer` as a regular transformer will get...