HeckmanEM               Fit the Normal or Student-t Heckman Selectio
                        model.
HeckmanEM.criteria      Model selection criteria for the Heckman
                        Selectio model
HeckmanEM.envelope      Envelope for the Heckman Selectio model.
HeckmanEM.infomat       Standard error estimation for the Heckman
                        Selectio model by the Information Matrix.
rHeckman                Generate data from the Heckman Selectio model
                        (Normal or Student-t).
