请问用op法求解tfp,exit怎么求的
下面的程序不明白


. sort firmid year
. by firmid: gen has_gaps = 1 if year[_n-1] != year-1 & _n != 1
. sort firmid has_gaps
. by firmid: replace has_gaps = 1 if has_gaps[_n-1] == 1
. replace has_gaps = 0 if has_gaps == .
. by firmid: generate exit = survivor == 0 & has95 == 0 & has_gaps != 1
> & _n == _N
.replace exit = 0 if exit == 1 & year == 2002