There's no "formal" contract in either place but people running on Fly Machines expect that there's nothing at all between them and the kernel, and we don't have that expectation in Sprites; we can do whatever we want. :)

I don't want to get too far into the rest of the details only because I'm writing this up for next week. They're not that interesting technically, but they're a really big deal for us in other ways.

Great, i look forward to reading it.