Starting homework3.
Last problem of hw2 unfinished.
At this point, I am not 100% sure why hw3 problem 2 and 3 work.
Refatored 2 and 3 so that they are shorter and avoids unnecessary value binding.
foldl is short for List.foldl.
Work towards problem 6.
first answer and all answer
unfinished.
todo: the last part.