help with S4 objects: trying to use a "link-glm" as a class in an object definition

Hi,

I'm trying to make a new S4 object with a slot for a "link-glm" object. R doesn't like me have a slot of class "link-glm"

class(make.link("probit"))

[1] "link-glm"

setClass("a",representation(item="link-glm"))

[1] "a"
Warning message:
undefined slot classes in definition of "a": item(class "link-glm") 

fa <- function() {

+   new("a",item=make.link("probit"))
+ }> 

fa()

Error in validObject(.Object) : 
  invalid class "a" object: undefined class for slot "item" ("link-glm")

# and a link-glm looks like a list to me, so I thought I would tell R it is a list and see what happens

setClass("b",representation(item="list"))

[1] "b"

fb <- function() {

+   new("b",item=make.link("probit"))
+ }

fb()

Error in validObject(.Object) : 
  invalid class "b" object: invalid object for slot "item" in class "b": got class "link-glm", should be or extend class "list"

Any advice?

Regards,
Paul Bailey
Ph.D. candidate
Department of Economics
University of Maryland

###### raw code #####
setClass("a",representation(item="link-glm"))
fa <- function() {
  new("a",item=make.link("probit"))
}
fa()
setClass("b",representation(item="list"))
fb <- function() {
  new("b",item=make.link("probit"))
}
fb()
###########