question about corStruct

["Ing. Marek Brabec PhD" <mbrabec@szu.cz>]

dear list,
I am wondering if one can find examples and/or more detailed 
descriptions of modifications needed when going beyond standard 
corStruct classes (i.e. those already provided for use in lme/nlme)? 
When I looked at pages 238-239 of Pinheiro/Bates (2000): Mixed-effects 
models in S and S-plus, I found that I would need a bit more explicit 
guidance what to do for implementing a new correlation structure than 
that presented there.
I am thinking e.g. of random walk as of a non-stationary example 
corStruct to implement.
Thanks for any help and/or pointing me to a source of general instructions.
All the Best
Marek Brabec