input: def insertion_sort(arr):
      """
      Perform insertion sort on a list of n distinct integers.
      
      >>> insertion_sort([3, 2, 1])
      [1, 2, 3]
      >>> insertion_sort([5, 6, 4, 7, 3, 9])
      [3, 4, 5, 6, 7, 9]
      """
      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