用STATA帮助的例子操作的，但还是有报错。

Examples
    Setup
        . webuse abdata
    Basic model with strictly exogenous covariates and two lags of the the
    dependent variable
        . xtdpdsys n l(0/1).w l(0/2).(k ys) yr1980-yr1984, lags(2)
    Same model with a robust VCE
        . xtdpdsys n l(0/1).w l(0/2).(k ys) yr1980-yr1984, lags(2)
            vce(robust)
    Twostep estimator of the same model with a robust VCE
        . xtdpdsys n l(0/1).w l(0/2).(k ys) yr1980-yr1984, lags(2) twostep
            vce(robust)

总是出现：
unrecognized command:  is_svy
r(199);

望请大侠相助，不胜感谢！