input: Write a python function to reduce an array by summing all of its elements.
  assert reduce_arr([1, 2, 3, 4, 5]) == 15
  assert reduce_arr([10, 20, 30]) == 60
  assert reduce_arr([-1, -2, -3, -4]) == -10
output: def reduce_arr(arr):
      return sum(arr)