If you could create a punctuation mark, what would its function be and what would it look like?

That's from Hudson Collins, loyal MR reader. I've always liked the chess marks "!?" and "?!" and wondered why they weren't used in standard English. The former refers to a startling move which is uncertain in merit and the latter refers to a dubious move which creates difficult to handle complications.

Yes! Yes!

Tyler Cowen at Marginal Revolution