input: def print_unique(my_list):
      """Print unique elements of a given list.
      >>> print_unique([1, 2, 2, 3, 4, 4, 5])
      1
      2
      3
      4
      5
      >>> print_unique(['a', 'b', 'b', 'c', 'c'])
      a
      b
      c
      """
      seen = set()
      for x in my_list:
          if x not in seen:
              print(x)
              seen.add(x)
output:     seen = set()
      for x in my_list:
          if x not in seen:
              print(x)
              seen.add(x)