input: def print_unique(my_list):
      """Prints the unique elements of a given list, each on a new line.
      >>> print_unique([1, 2, 2, 3, 4, 4])
      1
      2
      3
      4
      >>> print_unique(['a', 'b', 'b', 'a', 'c'])
      a
      b
      c
      """
output:     seen = set()
      for item in my_list:
          if item not in seen:
              print(item)
              seen.add(item)