input: def insertion_sort(arr):
      for i in range(1, len(arr)): 
          key = arr[i] 
          j = i-1
          while j >= 0 and key < arr[j] : 
                  arr[j + 1] = arr[j] 
                  j -= 1
          arr[j + 1] = key
  
  
  # Test cases
  assert insertion_sort([5, 3, 8, 6, 7, 2]) == [2, 3, 5, 6, 7, 8]
  assert insertion_sort([12, 11, 13, 5, 6]) == [5, 6, 11, 12, 13]
  assert insertion_sort([]) == []
  assert insertion_sort([1]) == [1]
  assert insertion_sort([1, 2, 3, 4, 5]) == [1, 2, 3, 4, 5]
output:     return arr