cdax

  fn sum(n):
    if n == 0:
      return 0
    else:
      return n + sum(n - 1)    // A

  fn sum_tail(n, pre_sum):
    if n == 0:
      return pre_sum
    return sum_tail(n - 1, pre_sum + n)    // B
    
  fn sum(n):
    return sum_tail(n, 0)