Diva遇到麻烦！！求救！！！

mujise

验证规则里有这样的语句：
checkPullUp( )
checkPullDown( )
/* Checks all nets for a path from the net through devices to nets
identified as power nets. Then checks all nets for a path from
the net through devices to nets identified as ground nets. */
两个语句的解释相同的，那两者究竟有什么区别？这两句话有什么用，是防止什么情况发生呢？