ch1 sum: stdio and ocamlbuild
ch3 destutter: refactor
ch4 freq: build tools and main
ch4 freq: refactor into modules
ch4 freq: abstract interfaces, interface generator, and ocamldoc
ch4 freq: optimize
ch4 freq: concrete signature
ch4: nested modules
ch4 ext_list: extending modules
Real World OCaml Ch1-4