input: Write a python function to sum all the elements in a list.
  assert sum_list([1, 2, 3, 4, 5]) == 15
  assert sum_list([10, 20, 30, 40]) == 100
  assert sum_list([-1, -2, -3, -4]) == -10
output: def sum_list(lst):
      return sum(lst)