Extending Point class problem

Hi,

I’m trying to make a Vector class which will provide some basic mathematical methods. I want to have this class extend the Point class, so it can be used in functions like <DisplayObject>.globalToLocal(pt: Point)

However, when I try to make a get/set method for the x or y values, I get this error:
1023: Incompatible override.

Is there any way around this ?