ex 3.20 - done
ex 3.21 - done
ex 3.22 - done
ex 3.23 - done
avoid foldl confusion
remove unused
mutural recursive working
ex 3.24 - prototype in racket
ex 3.24 - done
rename my own version of `foldl` to `my-fold`

the argument list is different
ex 3.25 - working
ex 3.25 - done
ex 3.26 - done
ex 3.27 - done
ex 3.28 - ds version
ex 3.28 - done
ex 3.38 - fix wrong impl
ex 3.29 - done
ex 3.30 - done
ex 3.31 - done