Suppose that sum(n) is defined as follows.
  case sum(n) = 1              when n == 1
  case sum(n) = sum(n-1) + n
You would like to check that sum(n) = 1 + 2 + ... + n by trying it for sum(5). Do an evaluation of sum(5). But assume that sum(k) = 1 + 2 + ... + k for all values of k that are less than 5.