input: Write a function to sum the elements of a list recursively.
  assert sum_list([1, 2, 3, 4, 5]) == 15
  assert sum_list([10, 20, 30, 40]) == 100
  assert sum_list([0, -1, -2, 3]) == 0
output: def sum_list(lst):
      if not lst:
          return 0
      return lst[0] + sum_list(lst[1:])