ROE、DAR 是内生变量，想采用广义矩估计法( GMM) 对联立方程进行估计，数据是一年的，求教如何实现？
ROE = c0 + c1DAR + c2GROW + c3TOP1 + c4TOP1sq +c5SIZE + c6BOARD + c7CONCURi + Ui
DAR = c0 + c1ROE + c2GROW + c3TOP1 + c4TOP1sq +c5SIZE + c6BOARD + c7CONCURi + Ui