ActionScript 2.0 :: DragOver - Drawing White Line By Erasing Dots

I have the raster of dots on white background. I would like it when I DragOver each dot it becomes white so it looks like you are actually drawing white line by erasing the dots. Ive somehow found the way how to do it by making it transparent when I rollover. But it doesnt work with dragover. How to do it only when I hold down the mouse button and drag my mouse over the dots?

This is the script assigned to MC with one black dot (I copied this MC with one dot into another MC and multiplied it to have the raster):
this.onRollOver = function () {
this._alpha = 0;
} this.onPress = function () {
this._alpha = 50;
}

I've tried also this, but it doesn't work:
var erasepoint = false;
this.onMouseDown = function(){erasepoint = true;}
this.onMouseUp = function(){erasepoint = false;}
this.onRollOver = function () {
if(erasepoint){
this._alpha = 0;
}}