input: Write a function to calculate the total sum of all elements in a given list. The function should take a list as input and return the sum of its elements.
  assert sum_elements([1, 9, 3, 5])==18
  assert sum_elements([0, 0, 0, 0])==0
  assert sum_elements([-1, 1, -1, 1])==0
  assert sum_elements([10, -5, 15, -5])==15
  assert sum_elements([])==0
output: def sum_elements(lst):
      return sum(lst)