input: 
  def insertion_sort(arr):
      """Sort a list of n distinct integers using the insertion sort method.
  
      >>> insertion_sort([12, 11, 13, 5, 6])
      [5, 6, 11, 12, 13]
      >>> insertion_sort([25, 17, 31, 13, 2])
      [2, 13, 17, 25, 31]
      """
      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
  
output:     return arr