[Rd] model.weights and model.offset: request for adjustment

[Ben Bolker]

   The model.weights() and model.offset() functions from the 'stats' 
package index possibly-missing elements of a data frame via $, e.g.

x$"(offset)"
x$"(weights)"

This returns NULL without comment when x is a data frame:

x <- data.frame(a=1)
x$"(offset)"  ## NULL
x$"(weights)"  ## NULL

However, when x is a tibble we get a warning as well:

x <- tibble::as_tibble(x)
x$"(offset)"
## NULL
## Warning message:
## Unknown or uninitialised column: `(offset)`.

    I know it's not R-core's responsibility to manage forward 
compatibility with tibbles, but in this case [[-indexing would seem to 
be better practice in any case.

   Might a patch be accepted ... ?

   cheers
    Ben Bolker