Write a function called gcd that takes two integer parameters and that uses Euclids algorithm to compute and return the greatest common divisor of the two numbers.

Exercise 5.5 (This exercise is based on page 44 of Ableson and Sussman's Structure and Interpre- tation of Computer Programs.) The following algorithm is known as Euclid's Algorithm because it appears in Euclid's Elements (Book 7, ca. 300 B.C.). It may be the oldest nontrivial algorithm. The algorithm is based on the observation that, if r is the remainder when a is divided and r. Thus we can use the equation gcd(a, b) gcd(b, r) to successively reduce the problem of computing a GCD to the problem of computing the GCD of smaller and smaller pairs of integers. For example., gcd(36, 20) = gcd(20, 16) = gcd(16,4) = gcd(4,0) = 4