input: Write a function to find the sum of elements of a list using a single line of code. If the given list is empty, return 0.
  assert sum_elements([1, 2, 3, 4])==10
  assert sum_elements([-1, 0, 1])==0
  assert sum_elements([5, 5, 5, 5])==20
  assert sum_elements([])==0
output: def sum_elements(lst):
      return sum(lst) if lst else 0