About '{:{fmt}}' and other wacky combinations, like '{.__doc__}':

It's much easier and cleaner in the code to allow these cases than it
would be to disallow them. And I rather like the '{:{fmt}}' example!

I suggest leaving them in. They might be useful, and it makes the
implementation cleaner. I think it will be easier to document, as well.
There are no special cases to describe: If the field name is omitted,
it's auto-numbered with a sequential index number. You can't mix
auto-numbering and manual specification of indexes.

As for string.Formatter, I agree we should leave it alone. I'd be
surprised if anyone is actually using it, anyway. But I'd love to hear
otherwise.