Prevents a class from being applied to a DOM element twice.
Class.Occlude = new Class({
occlude: function(property, element){
element = document.id(element || this.element);
var instance = element.retrieve(property || this.property);
if (instance && this.occluded != null)
return this.occluded = instance;
this.occluded = false;
element.store(property || this.property, this);
return this.occluded;
}
});