input: def insertion_sort(arr):
      """ Sort a list of n distinct integers using insertion sort.
      >>> insertion_sort([3, 2, 1])
      [1, 2, 3]
      >>> insertion_sort([4, 7, 9, 2, 5])
      [2, 4, 5, 7, 9]
      """
      for i in range(1, len(arr)): 
    
          key = arr[i] 
    
          # Move elements of arr[0..i-1], 
          # that are greater than key, 
          # to one position ahead of 
          # their current position 
          j = i-1
          while j >= 0 and key < arr[j] : 
                  arr[j + 1] = arr[j] 
                  j -= 1
          arr[j + 1] = key
output:     return arr