Based on the setting of the problem, this would only prevent the user from getting in when they, in actuality, have a valid ticket, but would not allow them to get in with a ticket that has been used and is actually invalid. No?

This would allow you change the value of used as much as you want, as long as the useTicket method hasn't been called yet. So you can have a ticket that says it's used because you changed it, but it's not really used - you can change it back because you haven't called the useTicket method and it's not frozen.

Well done! This should be at the top!