input: Write a function to calculate the total sum of all elements in the given list. If the list is empty, return 0.
  assert sum_elements([1, 9, 3, 5])==18
  assert sum_elements([-1, 2, 3, -4])==0
  assert sum_elements([])==0
  assert sum_elements([10])==10
output: def sum_elements(lst):
      return sum(lst)