the backbone function for the multilevelIV function
multilevelGMM(formula, endoVar, data = NULL)
an object of type 'formula': a symbolic description of the model to be fitted.
a matrix or data frame containing the variables assumed to be endogenous.
optional data frame or list containing the variables of the model.
returns the instrumental variables used in estimating the coefficients, variance-covariance matrix, the weight matrix used in estimating the model, etc.