[R] Defining binary indexing operators

[Ali -]

> > >
> > > Here is an example. Note that $ does not evaluate y so you have
> > >to do it yourself:
> > >
> > >x <- structure(3, class = "myclass")
> > >y <- 5
> > >foo <- function(x,y) x+y
> > >"$.myclass" <- function(x, i) { i <- eval.parent(parse(text=i)); foo(x,
> > i)
> > >}
> > >x$y # structure(8, class = "myclass")
> >

If I got it right, in the above example you provided '$' is defined as a 
method for a S3 class. How is it possible to do the same with a S4 class. If 
this is not possible, what is the best way to define the '$' operator whose 
first arguments is a S4 object and doesn't overwrite the global definition?