A drag and drop example from a clueless coder

I'd like to take the following JavaScript (http://cyberdummy.co.uk/test/dd.php) and limit the list items to their groups - so the list items may move vertically, but not between groups.

Since the blog entry seems pretty out-dated and un-answered, I was hoping someone in here might be able to give me an insight.

Cheers, Dave

Coordinate.prototype.reposition = function(element) {
element.style["top"] = this.y + "px";
// element.style["left"] = this.x + "px";
}

I hate to sound unappreciative, but that is far from being helpful. Where would this code go? How would it stop the user from switching list items from one group to another?