ex 3.7 - copy code
ex 3.7 - some plans
ex 3.7 - slight mod, make code clearer
ex 3.7 - obj-holder
ex 3.7 - use obj-holder
ex 3.7 - make `joint` an internal call
ex 3.7 - done
ex 3.8 - first attempt
ex 3.8 - done
3.2 - taking notes
3.2 - done
3.2.1 - done
3.2.1 - add an example
3.2.2 - done
ex 3.9 - half way
ex 3.9 - done
3.2.3 - done
ex 3.10 - 1.desugar
ex 3.10 - step 2,3
ex 3.10 - plan to use LaTeX