ex 4.51 - impl permanent-set!
ex 4.51 - done
amb-eval - new testcase for set!
ex 4.52 - impl
ex 4.52 - not sure about what happen if try-again
ex 4.52 - need "even?"
my-eval - primitive even? and odd?
ex 4.52 - example didn't work
ex 4.52 - now it's working
ex 4.52 - if-fail as derived expression
ex 4.52 - done
ex 4.{51,52} - share slot handlers
ex 4.53 - my prediction
ex 4.53 - putting libs & progs together

`cond` is missing
my-eval - expose if->cond
amb-eval - add slot "cond"
amb-eval - "cond" slot is done
my-eval - lift remainder and quotient
ex 4.53 - working program
ex 4.54 - skeleton code