oops, fixed exercise 2.8. subtraction is just addition of the negative!
oops, stupid car/cdr
Exercise 2.9 - first test code, which let me fix moar stupid bugs in 2.8
Exercise 2.10 - a minor modification to (div-interval)
started mul-interval-v2 - done with the non-straddle-0 cases
refactored +- cross case into shared routine; todo - same for straddle case