[R] need help getting started writing a new varFunc class for lme()

[Sarah Holte]

Hello - I've been using R for years, but have always been able to find 
what I need already available.  Now I find that I would like to write a 
new varFunc class for the lme() or nlme() packages.  There is some 
guidance for this in Problem 4 Chapter 5 of Bates and Pinheiro Mixed 
Effects Models in S and S-Plus.  However, I find that I am unable to 
even get started and so have just purchased John Chambers Book - 
Programming with Data, A Guide to the S Language.

Any specific tips about how to write a new varFunc class (I want one 
that models the variance of longitudinal correlated data as a logistic 
function of a covariate) or general tips or references to sections in 
Chambers book about the steps mentioned in Problem 4, Chapter 5 of Bates 
and Pinheiro (eg. write a constructor, constructor arguements (value and 
form), write an initialize method, etc) would be much appreciated.   I 
did find a few threads in the archives discussing this problem, but they 
were already way beyond my understanding of how to execute writing this 
new new varFunc class.

Thanks so much - Sarah Holte