Section8.2. Environment Options

1. hook是个什么东西?干什么用的?

2. exercise/pre-hook = {code} (initially empty)
   The code for the pre exercise hook for exercises of the type “exercise”.
   exercise/begin-hook = {code} (initially empty)
   The code for the begin exercise hook for exercises of the type “exercise”.
   exercise/end-hook = {code} (initially empty)
   The code for the end exercise hook for exercises of the type “exercise”.
   exercise/post-hook = {code} (initially empty)
   The code for the post exercise hook for exercises of the type “exercise”.
   这四个命令是干什么的?有什么作用?
   solution环境也有这四个命令.