hw2 checkpoint 1

initial commit + exercise 1
hw2 checkpoint 2
hw2 checkpoint 3

finished exercise 2
hw2 checkpoint 4

modified exercise 2, initial commit ex3
hw2 checkpoint 5

finished exercise 3
hw2 checkpoint 6
hw2 checkpoint 7

semi finished es4
hw2 checkpoint 8

work finished
hw2 checkpoint 9

final commit