Exercises 2.77-80 play with the generic arithmetic system. exercise 77 is just the beginning
added comment
added some more tests to double-check my understanding
Exercise 2.78 - hacking the primitives to allow simple scheme numbers to be used directly, without calling (make-scheme-number
Exercise 2.79: adding a new function seems to require modifying the 'base class'
made dead ends more self-evidently dead
added some notes