ex 4.11 - done
let me skip 2 exercises: ex 4.12 and ex 4.13
ex 4.14 - done
ex 4.15, 4.16 - done
ex 4.17 - rules
ex 4.17 - multiarg procedures
ex 4.17 - multiarg support for letrec
ex 4.17 - done
ex 4.18 - skipped
ex 4.19 - almost done
ex 4.19 - done
ex 4.20 - step 1. ref as valid expval

just for impl purpose, I know ExpVal and DenVal are
different stories, but here it seems to be the easiest way
of getting things done.
ex 4.20 - done
ex 4.21 - done
ex 4.22 - define statements
ex 4.22 - done, plan to bring test facility back.
ex 4.22 - bug fix
ex 4.22 - new facility - printer
my-eval - define - done / bug fix
my-eval - use 'ok to mark things done