[Rd] how to use update if method for formula is not in namespace?

[Liaw, Andy]

Dear R-devel,

I've noticed that if the method for formula is not exported in NAMESPACE,
then update() would fail.  As an example:

> library(e1071)
> data(iris)
> iris.svm <- svm(Species ~ ., data=iris)
> update(iris.svm)
Error in eval(expr, envir, enclos) : couldn't find function "svm.formula"

The same thing happens with randomForest, because randomForest.formula() was
not exported.  Is this the intended behavior of update()?  What would be the
preferred work-around?

Best,
Andy