[R] How to add new corStruct class in lme?

I was trying to fit a linear mixed model with a user specified correlation structure with lme (package nlme). In the book of Pinheiro and Bates (2000), it was mentioned that new corStruct classes can be added, and the corAR1 constructor and methods can serve as templates. But I am just too dumb t

[R] question of correlation structure in gls

Dear R users, I want to fit a model with within-subject correlations but with no random effects. So I could use gls to set correlation=corSymm(~1 | group). Here the problem I encountered is that I have 90 groups, and for the first, second and third 30 groups I want them to have three different u