input: Write a python function to sum elements of two lists at the same index.
  assert sum_elements([1, 2, 3], [4, 5, 6]) == [5, 7, 9]
  assert sum_elements([10, 20, 30], [1, 2, 3]) == [11, 22, 33]
  assert sum_elements([5, 15, 25], [10, 20, 30]) == [15, 35, 55]
output: def sum_elements(list1, list2):
      return [x + y for x, y in zip(list1, list2)]