The following function sum(n) yields 1 + 2 + ... + n,
the sum of the first n positive integers. For
example, sum(3) = 1 + 2 + 3 = 6 and sum(5) = 1 + 2 + 3 + 4 + 5 = 15.
case sum(n) = 1 when n == 1
case sum(n) = sum(n-1) + n
Do a step-by-step evaluation of sum(4).