https://weils.net/blog/2014/02/09/left-4-dead-2-expert-mode-%e6%83%a1%e9%9d%88%e5%8b%a2%e5%8a%9b-%e5%b0%88%e5%ae%b6%e6%a8%a1%e5%bc%8f-%e8%aa%aa%e6%98%8e/