Python 3!

The greatest common divisor (GCD) of a and b is the largest number that divides both of them with no remainder. One way to find the GCD of two numbers is based on the observation that if r is the remainder when a is divided by b, then gcd(a, b) = gcd(b,r). As a base case, we can use gcd(a, 0) = a. Write a function called gcd that takes parameters a and b and returns their greatest common divisor. Credit: This exercise is based on an example from Abelson and Sussmans Structure and Interpretation of Computer Programs.