[Rd] update.default bugfix (PR#3288)

[Thomas Lumley]

On Wed, 18 Jun 2003 minka at stat.cmu.edu wrote:

> According to the man page for formula, "a formula object has an associated
> environment".  However, update.default doesn't use this environment, which
> creates problems like the following:
>
>   make.model <- function(x) { lm(medv~.,x) }
>   library(MASS)
>   data(Boston)
>   fit = make.model(Boston)
>   fit = update(fit,".~.-crim")
>   # Object "x" not found

Update isn't going to satisfy everyone because people want it to do
incompatible things.  However, I don't think this is one of the things it
should be able to do.

In general the environment of a formula is used to look up variables in
the formula, not the data= argument.  (It's also used to look up the
weights= and subset= arguments, but that is a horrible wart that we would
like to excise).

	-thomas