[R] Re: creating new varFunc classes in nlme .. error: "Don't know how to get coefficients for .. object"

[J]

Ah!! After way too many hours of work Ive answered my
own question.  To get varExp2, a copy of varExp, to
work as a new varFunc, one needs to use this sort of
syntax:

update.varExp2 <- function (object, data, ...)
{
    print ("enter update")
    val <- NextMethod()
    if (length(val) == 0) {
        aux <- coef(val, allCoef = TRUE)
        if (!is.null(grps <- getGroups(val))) {
            aux <- aux[grps]
        }
        attr(val, "logLik") <- sum(log(attr(val,
"weights") <- exp(-aux *
            getCovariate(val))))
    }
    val
}
setMethod("update", "varExp2", update.varExp2)

And then do the same for all methods used by varExp
(see methods(class="varExp")).  When writing the
setMethod for coef<-.varExp2, use
cat("coef<-.varExp2") in the statement.  Or at least
that worked for me.

But I still have an unanswered question from the other
day (see [R] two questions on nlme: error messages and
nested variance).  What is the best way in nlme to
model the variance when the variance of the variance
is not constant but is dependent on a covariate?  

Thanks.  John