erasing an object

Question

Hey i'm trying to erase a movieclip but it doesnt want to work. My code is:

        private function setupErase():void{
        var toolsize = 20;// size used for tools (width = height)
        var tooloffset = toolsize/2;//
        var bitmap:Bitmap=new Bitmap();
        var bd:BitmapData=new BitmapData(filth.width,filth.height,true,0x00FFFFFF);
        var basepoint = new flash.geom.Point(0, 0);
        bd.draw(filth);

        bitmap.bitmapData=bd;
        addChild(bitmap);
        bitmap.x = filth.x;
        bitmap.y = filth.y;
        removeChild(filth);
        var erasebmp:BitmapData = new BitmapData(toolsize, toolsize, true, 0xFFFFFFFF);
        var rect:Rectangle=new Rectangle(erasebmp.width,erasebmp.height);
        // then draw the shape (black) into the bitmap
        erasebmp.draw(sponge);
        // copyChannel is used to convert the shape of the tool
        // just added (drawn) into erasebmp into it's alpha channel
        erasebmp.copyChannel(erasebmp, erasebmp.getColorBoundsRect(0xFFFFFF,0,true), basepoint, 1, 8);
        var pointer_mc:MovieClip=new MovieClip();
        pointer_mc.addChild(sponge)
        sponge.x=-sponge.width/2
        sponge.y=-sponge.height/2
        addChild(pointer_mc);
        //listeners
        stage.addEventListener(MouseEvent.MOUSE_MOVE,onMove);
        stage.addEventListener(MouseEvent.MOUSE_DOWN,onDown);
        stage.addEventListener(MouseEvent.MOUSE_UP,onUp);
        var erasing:Boolean=false;

        function onDown(ev:MouseEvent) {
            erasing=true;
            erase();
        }
        function onUp(ev:MouseEvent) {
            erasing=false;
        }

        function onMove(ev:MouseEvent) {
            pointer_mc.x=mouseX;
            pointer_mc.y=mouseY;
            if (erasing) {
                erase();
            }
        }

        function erase() {
            // devise an offset based on the mouse's
            // position in pointer_mc and tooloffset
            var offset = new flash.geom.Point(mouseX-(pointer_mc.width/2) , mouseY-(pointer_mc.width/2) );

            // devise a drawing rectangle the size of the
            // the drawing tools at the offset's location
            var drawRect = new flash.geom.Rectangle(offset.x, offset.y, toolsize, toolsize);

            // next, copy pixels from the drawbmp back onto itself
            // (makes for no change) but use erasebmp as the alphaBitmap
            // this will add the alpha channel information from erasebmp
            // into drawbmp erasing pixels from drawbmp in the shape of erasebmp
            bd.copyPixels(bd, drawRect, offset, erasebmp, basepoint, false);
        }
    }

"filth" is what needs to be erased and "sponge" is what needs to erase it. Anyone see where the problem is?