input: Write a python function to sum all the elements of a list.
  assert sum_of_list([1, 2, 3, 4, 5]) == 15
  assert sum_of_list([10, 20, 30]) == 60
  assert sum_of_list([-1, -2, -3, -4]) == -10
output: def sum_of_list(lst):
      return sum(lst)