> what is the advantage of having this as one function rather than end_map
> and end_list? Indeed, my choice (but I think most other people would
> disagree) would be to have both functions but still keep state as this
> currently does and then throw an error if the code closes the wrong
> thing.

There's probably no advantage, one way or the other is fine, I'd say.
I've thought about introducing checks into the formatter functions, to
raise errors for improper closing, map_key not inside a map and things
like that, I just wasn't sure that would be acceptable.

> A second question: do you have an implementation in this style for
> s-expressions. I find it hard to tell whether the interface is right
> with just a single example. Even a completely hacky not ready for review
> example would be helpful.

See the attached patch :)

Thanks for the suggestions!

Peter