[R] lme correlation structures

[Gareth Hughes]

Hi all,

I've been using SAS proc mixed to fit linear mixed models and would
like to be able to fit the same models in R. Two things in particular:

1) I have longitudinal data and wish to allow for different repeated
measures covariance parameter estimates for different groups (men and
women), each covariance matrix having the same structure. In proc
mixed this would be done by specifying group= in the REPEATED
statement. Is this simple to do in R? (I've tried form=~time|indv/sex
for example but this doesn't seem to do the job).

2) I've read that other correlation structures can be specified. Does
anyone have any examples of how toeplitz or (first-order)
ante-dependence structures can be specified?

Many thanks,

Gareth