input: Write a python function to calculate the maximum sum of an array, possibly including a forbidden index.
  assert max_sum([3, 2, 5, 1, 6], 2) == 12
  assert max_sum([1, 3, 2, 5, 8], 0) == 18
  assert max_sum([4, 5, 6, 7, 8], 3) == 30
output: def max_sum(arr, forbidden_index):
      return sum(arr) - arr[forbidden_index]