Barry: Sure, the docs example was just a quick write-up, you can word it however you want!

Guido: Pretty much, except the other way around (when prune is False, i.e. "don't remove empty strings").

The attached patch exposes the behaviour (it's identical to last night's, but I'm re-uploading it as an unrelated file went in), except that the `prune` argument isn't keyword-only (I didn't know how to do this, and didn't bother searching for just a proof-of-concept).