input: Write a python function to calculate the cumulative sum of all elements in a list.
  assert sum_of_all([2, 3, 4, 5, 6]) == 35
  assert sum_of_all([1, 2, 3, 4]) == 20
  assert sum_of_all([10, 20, 30]) == 100
output: def sum_of_all(lst):
      return sum(sum(lst[:i+1]) for i in range(len(lst)))