> What should the format specifier mini-language for complex numbers look
> like?
> Should it look like the existing mini-language for floats, but have
> the format specified twice, with some sort of delimiter?

This sounds clumsy to me.  I'd guess that in most uses you'd want the
same format for both pieces.

> Or just specified once, and use that for both parts?

That doesn't sound unreasonable.  But there might need to be some
thinking about exactly what a '+' modifier means, or how you pad with
zeros on the left when you've got two pieces to pad.

It seems simplest just to tell people to format the real and imaginary
parts by hand.  As it isn't totally obvious how to do this (e.g.,
remembering the '+' for the imaginary part), perhaps there should be a
recipe in the docs somewhere?