require(nlme)

help(corSymm, help_type="html")

fm1Orth.gls <- gls(distance ~ Sex * I(age - 11), Orthodont,
                   correlation = corSymm(form = ~ 1 | Subject),
                   weights = varIdent(form = ~ 1 | age))

summary(fm1Orth.gls)

À disposição.
Walmes.