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, 3, 2, 1])==None
  assert print_unique([4, 5, 6, 5, 4, 7])==None
  assert print_unique([1, 2, 3, 4, 5])==None
  assert print_unique([8, 8, 9, 9, 10, 10])==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)