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, 2, 3, 4, 4])#output should be 1, 2, 3, 4
  assert print_unique([5, 5, 5, 5])#output should be 5
  assert print_unique([])#output should be no output
  assert print_unique([7, 8, 9, 7, 8, 9])#output should be 7, 8, 9
output: def print_unique(my_list):
      seen = set()
      for x in my_list:
          if x not in seen:
              print(x)
              seen.add(x)