xtabond2  ddip roa l.ldip l.ddip duofut dofuy dofup  leta du9* dupav1 dupav3 dupav4,
iv(du9*  leta) iv(l.ldip l.duofut l.dofuy l.dofup, eq(level))
gmm(l.ddip duofut dofuy dofup roa,collapse) gmm( dupav1 dupav3 dupav4, collapse
eq(diff)) robust twostep;

是怎么处理的？ iv gmm是怎么回事?