. xtset legalcode year
       panel variable:  legalcode (unbalanced)
        time variable:  year, 2001 to 2007, but with gaps
                delta:  1 unit
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xtabond2 y x1 x2 x3 x4 x5 x6  L1.y L1.x1 L1.x2,gmm(y x1 x2 ,lag(2 2)) iv(lx3 x4 x5 x6  ) robust small

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做出的结果数据量（Number of obs ）少了很多，估计是自动处理成平衡面板做的，怎么用非平衡面板做gmm啊