ch03 - begin
3.1 - taking notes
3.1.1 - two impl of withdraw
3.1.1 - make-withdraw
3.1.1 - make-account impl
3.1.1 - make-account usage
3.1.1 - done
ex 3.1 - make-accumulator
ex 3.1 - done
ex 3.2 - start and new challenge
ex 3.2 - impl
ex 3.2 - done
ex 3.3 - impl
ex 3.3 - done
ex 3.4 - impl a trigger
ex 3.4 - make-account extension impl
ex 3.4 - done
3.1.2 - some example regarding mit-scheme rnd impl
3.1.2 - monte-carlo
3.1.2 - monte-carlo, rand-update