This is an internally called function used to fits a linear mixed-effects model; see lme
.
Laird, N.M. and Ware, J.H. (1982) Random-Effects Models for Longitudinal Data, Biometrics, 38, 963<U+2013>974.
Pinheiro, J.C. and Bates., D.M. (1996) Unconstrained Parametrizations for Variance-Covariance Matrices, Statistics and Computing, 6, 289<U+2013>296.
Pinheiro, J.C., and Bates, D.M. (2000) Mixed-Effects Models in S and S-PLUS, Springer.