数据如下：
input int year str1 firm int firmid
2011 "A" 556
2012 ""  556
2013 "A" 556
2014 ""  556
2015 "A" 556
2011 "B" 558
2012 "B" 558
2013 ""  558
2014 ""  558
2015 ""  558
end
我想把以上数据变为
input int year str1 firm int firmid
2011 "A" 556
2012 "A" 556
2013 "A" 556
2014 "A" 556
2015 "A" 556
2011 "B" 558
2012 "B" 558
2013 "B" 558
2014 "B" 558
2015 "B" 558
end

即按照firmid相同的，将firm空缺值插补上
stata应该用什么命令，谢谢各位