ex 2.11 - done & remove unused
ex 2.12, ex 2.13
ex 2.14 - done
ex 2.15 - impl
ex 2.15 - test proc
ex 2.16 - done
ex 2.17 - impl 1
ex 2.17 - impl 2
ex 2.17 - done
chapter 4
4.1 metacircular evaluator
define eval
ex 2.18 - done
ex 2.19 - done
ex 2.20 - done
import `format`
ex 2.21 - done
ex 2.22 - done
transplant my auto-test code here
import `compose` and `flip`