these three models are replication in R of stata's
code available on the web site of the American Economic
Association. The estimation is complicated by the fact that
some linear constraints are imposed. The estimation was
performed using the maxLik package. As the Hessian is near
singular, the bread method for maxLik which use the vcov
method returns an error. Therefore, we use a new maxLik2
class and write specific llcont, estfun and bread methods
for this class.