Library class override events

I’m having a total brain fart right now because I haven’t dealt with AS2 library linkage classes in a while. I want to be able to let a designer assign an object in the library to a specific class and have that class listen for changed _x and _y values.

I have the class made up that extends MovieClip:

class CoordinateMonitor extends MovieClip {
   public function CoordinateMonitor () {}

But I draw a blank now on how I’m to monitor _x and _y values for the clip. I’m pretty sure I remember it being possible to do this and declaring an overriding function (“public function set _x(val:Number) { super._x = val; }”) does not work as intended.

Maybe someone can refresh my memory… give me direction?