input: Write a python function to test the Phone and Person classes interaction.
  assert john.phone.name == "iPhone"
  assert john.phone.owner == john
  assert john.use_phone() == "John is using iPhone"
  assert emma.phone.name == "Samsung"
  assert emma.phone.owner == emma
  assert emma.use_phone() == "Emma is using Samsung"
output: class Phone:
      def __init__(self, name, owner=None):
          self.name = name
          self.owner = owner
  
  class Person:
      def __init__(self, name, phone=None):
          self.name = name
          self.phone = phone
          if phone:
              phone.owner = self
      
      def use_phone(self):
          return f"{self.name} is using {self.phone.name}"
  
  # Test the interaction
  john = Person("John", Phone("iPhone"))
  emma = Person("Emma", Phone("Samsung"))
  
  assert john.phone.name == "iPhone"
  assert john.phone.owner == john
  assert john.use_phone() == "John is using iPhone"
  assert emma.phone.name == "Samsung"
  assert emma.phone.owner == emma
  assert emma.use_phone() == "Emma is using Samsung"