##use lists to get oo by contract
##by using class(ttt) <- "point"
##in the correct manner you can easily get S3 classes.

 plotxy <- function(x, col, xlim, ylim, ...) {
  xval <- Xpos(x)
  yval <- Ypos(x)
  plot(xval,yval, xlim=c(0,xval+1), ylim=c(0,yval+1), ...)
 }

 newXY.xy <- function(x,y) list(x=x,y=y)
 newXY.rt <- function(r, theta) list(x=r*cos(theta), y=r*sign(theta))

 newRT.xy <- function(x,y) list(r=sqrt(x^2+y^2), theta=tan(y/x))
 newRT.rt <- function(r, theta) list(r=r, theta=theta)


 pt1 <- newXY.xy(1,2)
 pt2 <- newRT.rt(4, .44)

##access via functions:

 Xpos.xy <- function(pt) pt$x
 Ypos.xy <- function(pt) pt$y

 Xpos.rt <- function(pt) pt$r*cos(pt$theta)
 Ypos.rt <- function(pt) pt$r*sin(pt$theta)


 Rval.rt <- function(pt) pt$r
 Tval.rt <- function(pt) pt$theta

 Rval.xy <- function(pt) sqrt(pt$x^2+pt$y^2)
 Tval.rt <- function(pt) tan(y/x)

##first suppose we are using xy points
##then Xpos and Ypos would have the following defs

 Xpos <- Xpos.xy
 Ypos <- Ypos.xy

 plotxy(pt1)

##suppose instead we were using r-theta
##then we would have defined Xpos and Ypos as

 Xpos <- Xpos.rt
 Ypos <- Ypos.rt

 plotxy(pt2)

##Notice that I did not need to change any code in plotxy