ex 3.40?
ex 3.40 - approach #1, modify env
move to new dir and make a new start
ex 3.40 - keep track of `letrec` mark
ex 3.40 - don't do that
ex 5.1 - procedural representaion, infrastructure
ex 5.1 - impl procedural representation
ex 5.1 - done
ex 5.3 - done
ex 5.2 - done
ex 5.4 - done
Merge branch 'eopl'