input: def print_unique(my_list):
      """
      Prints the unique elements of a given list.
      >>> print_unique([1, 2, 3, 4, 3, 2, 1])
      1
      2
      3
      4
      >>> print_unique(['a', 'b', 'c', 'b', 'a'])
      a
      b
      c
      """
output:     unique_elements = set(my_list)
      for element in sorted(unique_elements):
          print(element)