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, 4, 5, 5])==None
  # should print: 1 2 3 4 5
  
  assert print_unique([2, 4, 6, 8, 6, 4, 2])==None
  # should print: 2 4 6 8
  
  assert print_unique([])==None
  # should print: (nothing)
output: def print_unique(my_list):
      seen = set()
      for x in my_list:
          if x not in seen:
              print(x)
              seen.add(x)