startDrag = constrain to only y movements

programming flash
1

Can anybody help me with this?I am doing a ‘draggable’ mask but just want it to move on the y. Here is the code that is working so far:

onClipEvent (load) {
    startDrag (this, true);
    posx = _x ;
    posy = _y ;
}
onClipEvent (enterFrame) {
    back._x = posx - _x ;
    back._y = posy - _y;
}