implementing list primitives and stacks
ex 5.20 - some answers
5.20 - done
3.5 - some comments

Things are still missing, so we just simply lift
what we have for now and ignore the details.
simu - list primitive patch
simu & legacy-easy - add "not" primitive
ex 5.21 - controller

first version, not sure if it works
ex 5.21 - controller impl fix
ex 5.21 - simu - program
ex 5.21 - controller comments
ex 5.21 - tests on simu
ex 5.21 - iterative version controller
ex 5.21 - controller fix
ex 5.21 - simu - done
ex 5.21 - done