input: Write a function to improve the existing implementation of finding the sum of elements in a list. The function should use a more efficient method than iteration. If the input list is empty, the function should return 0.
  
  def sum_elements(arr):
      return sum(arr) # return sum of elements of arr with one line of code.
  
  assert sum_elements([1, 2, 3, 4, 5])==15
  assert sum_elements([-1, -2, -3, -4, -5])==-15
  assert sum_elements([0])==0
  assert sum_elements([])==0
output: def sum_elements(arr):
      return sum(arr)