input: Write a function that prints the unique elements of a given list.
  
  def print_unique(my_list):
      seen = set()
      for x in my_list:
          if x not in seen:
              print(x)
              seen.add(x)
  
  assert print_unique([1, 2, 2, 3, 4, 4])==None
  assert print_unique([5, 6, 5, 7, 6, 8, 9])==None
  assert print_unique([10, 10, 10, 10])==None
  assert print_unique([11, 12, 13, 12, 11])==None
  assert print_unique([])==None
output: def print_unique(my_list):
      seen = set()
      for x in my_list:
          if x not in seen:
              print(x)
              seen.add(x)