进行非参数回归时系统自动生成变量_d_Mean_incidence1dvaccination_p，然后就报错说已被定义。
drop掉该变量之后，重新运行还是报错。
有没有人知道这个怎么处理呀？
. npregress kernel incidence1 i.vaccination_policy i.testing_policy containment_index,vce(bootstrap, reps(100) seed(111))
(running npregress on estimation sample)
variable _d_Mean_incidence1dvaccination_p already defined
an error occurred when bootstrap executed npregress
r(110);