refactoring makes things MUCH clearer! still have the non-trivial double-straddle case to go. oh you crafty crafty ben bitdiddle
Exercise 2.11: mul-interval-v2 TDD done
Exercise 2.12: make-center-percent. didn't feel like checking this...
Exercise 2.13: small percentage tolerances are additive when you take the product. just a pencil/paper exercise?
Exercises 2.14-16 meh. done with section 2.1!