cd "C:\Documents and Settings\liang\桌面\xxxxxxx"
use xxxxxxx.dta,clear
gen obs=_n
tset obs
capture drop x1_1
gen x1_1=x1
replace x1_1=x1_1[_n-1] if x1_1==""
capture drop var_1
gen var_1=var
replace var_1=var_1[_n-1] if var_1==""

egen group=group(var_1 x1_1)

xtset group obs
capture drop obs_1
gen obs_1=.
replace obs_1=obs if var=="" & x1=="" & x1[_n+1]!=""

use xxxxxxx.dta,clear
gen obs=_n
gen obs_1=.
replace obs_1=obs if var[_n]=="" & x1[_n]=="" & x1[_n+1]!=""