Interval arithmetic

We built programs for adding (a, b) to (p, q) as described in SICP 2.1.4

I don't really remember this one. I think Andrey and I built a system in J -- in 45 minutes we only managed to write a single short line of code, but it solved the problem for us.

A good exercise, but tricky to described properly. Would be best to post some links ahead of time, with a clear description of the problem and some visual aids.