input: 
  def insertion_sort(arr):
      """ Sorts a list of n distinct integers using the insertion sort algorithm.
      >>> insertion_sort([12, 11, 13, 5, 6])
      [5, 6, 11, 12, 13]
      >>> insertion_sort([7, 4, 3, 8, 1])
      [1, 3, 4, 7, 8]
      """
      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