input: Write a python function to calculate the sum of all elements in a list.
  assert sum_elements([1, 2, 3, 4, 5]) == 15
  assert sum_elements([10, 20, 30, 40, 50]) == 150
  assert sum_elements([-1, -2, -3, -4, -5]) == -15
output: def sum_elements(lst):
      return sum(lst)