input: Write a function to print all unique elements from a given list. If the list is empty, print nothing.
  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, 6, 7, 8])==None
  assert print_unique([3, 3, 3, 3])==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)