How to cancel an effect

Hi all,

I have a situation where I want to display an error message, and have  
it fade away after a 2 second delay.  My problem is an impatient user  
could click buttons so fast that multiple Fade effects will be in  
play.  I believe script.aculo.us can supports canceling existing  
effects before starting a new one, but couldn't find any examples.

I tried something like this but it doesn't seem to work:

        var old_effect;

        if (old_effect) old_effect.cancel
        old_effect = new Effect.Fade($('error_msg'));

Am I close?

Thanks!
--Dave.