===== Testing GCD =====
GCD of 15 and 18 is 3
GCD of 18 and 15 is 3
GCD of 80 and 20 is 20
GCD of 20 and 80 is 20
GCD of 21 and 47 is 1
GCD of 47 and 21 is 1
GCD of 68 and 153 is 17
GCD of 153 and 68 is 17
GCD of 0 and 18 is 18
GCD of 18 and 0 is 18
GCD of 0 and 0 is 0
GCD of 0 and 0 is 0