ex 5.15
ex 5.16
ex 6.1
ex 6.2
ex 6.3 + bug fixed in nonNegativeInt
ex 6.4
ex 6.5
ex 6.6
ex 6.7
ex 6.7 updated after peeking at the solution (didn't understand the foldRight approach)
ex 6.8
ex 6.9
ex 6.10
ex 6.11
All code reformatted