input: Write a function to print 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, 3, 2, 1]) == None
  assert print_unique([4, 5, 4, 5, 6]) == None
  assert print_unique([7, 7, 7, 7]) == 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)