[R] S4 classes?

[Spencer Graves]

      The following example, extracted from pp. 38-42 of Chambers (1998) 
Programming with Data (Springer), works for me in S-Plus 6.2 but not R 
1.9.0pat, which returns the indicated error messages: 

 > setClass('track', representation(x="numeric", y="numeric"))
[1] "track"
 > tr1 <- new("track", x=1:3, y=4:6)
 > setMethod("plot",
+ signature(x="track", y="missing"),
+ function(x, y, ...)plot(x at x, x at y, ...)
+ )
[1] "plot"
 > plot(tr1)
Error in plot.window(xlim, ylim, log, asp, ...) :
        need finite xlim values
In addition: Warning messages:
1: no finite arguments to min; returning Inf
2: no finite arguments to max; returning -Inf
3: no finite arguments to min; returning Inf
4: no finite arguments to max; returning -Inf

      Am I missing something or is my copy of R 1.9.0pat corrupted?  
(This is under Windows 2000.) 

      Thanks Much!
      Spencer Graves