text: brianna = User(1, 'Brianna')
  mary = User(2, 'Mary')
  
  keyboard = brianna.sell_product('Keyboard', 'A nice mechanical keyboard', 100)
  print(keyboard.available)  # => True
  mary.buy_product(keyboard)
  print(keyboard.available)  # => False
  review = mary.write_review('This is the best keyboard ever!', keyboard)
  review in mary.reviews  # => True
  review in keyboard.reviews  # => True
  