input: Write a python function to find the sum of all elements in an array.
  assert sum_elements([2, 3, 4]) == 9
  assert sum_elements([10, 20, 30, 40]) == 100
  assert sum_elements([1, 1, 1, 1, 1, 1]) == 6
output: def sum_elements(arr):
      return sum(arr)