This function is not intended to be called by users and therefore not exported.
References
Searle, S.R, Casella, G., McCulloch, C.E. (1992), Variance Components, Wiley New York
Giesbrecht, F.G. and Burns, J.C. (1985), Two-Stage Analysis Based on a Mixed Model: Large-Sample
Asymptotic Theory and Small-Sample Simulation Results, Biometrics 41, p. 477-486