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 to figure out where are the mentioned templates, and it looks like they are not included in the book. Can anyone give me some hints on how to add in a new corStruct class? Thank you, JLY [[alternative HTML version deleted]]