R help - Jan 2010 - a question about the variance of growth model

Hi all:
In the growth model (Ref.Multilevel Modeling in R (2.3)),I have a question about
the variance:
 
On p.73, it says:"To model decreasing variance one can use the varExp
option. In cases where variance increases can use the varFixed option (see
Pinheiro & Bates, 2000 for details)."
 
My question is: if variance's change is not in a unique trend(for instance:
varance increase at first, and then decrease...) How to do with this situation?
 
Thanks a lot!
 
My best 
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