sample code of 3.5.1
chapter 3, exercise 50
sample code of 3.5.2
ex 3.53
ex 3.54
ex 3.55
ex3.56
exercise 3.57&3.58
exercise 3.59
exercise 3.60 untested
exercise 3.61, no test
ex 3.62 no test
sample code of 3.5.3
ex3.64, no test
ex3.65
sample code in infinite streams of pairs
exercise 3.67
exercise 3.69
ex 3.70
ex 3.71