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
  
  # Example:
  # >>> insertion_sort([64, 34, 25, 12, 22, 11, 90])
  # [11, 12, 22, 25, 34, 64, 90]
  
output:     return arr