code: function(fn, obj, overrideContext) {
  
          if (!fn) {
  throw new Error("Invalid callback for subscriber to '" + this.type + "'");
          }
  
          if (this.subscribeEvent) {
              this.subscribeEvent.fire(fn, obj, overrideContext);
          }
  
          var s = new YAHOO.util.Subscriber(fn, obj, overrideContext);
  
          if (this.fireOnce && this.fired) {
              this.notify(s, this.firedWith);
          } else {
              this.subscribers.push(s);
          }
      }
docs: Subscribes the caller to this event
  @method subscribe
  @param {Function} fn        The function to execute
  @param {Object}   obj       An object to be passed along when the event fires.
  overrideContext <boolean|Object> If true, the obj passed in becomes the execution
  context of the listener. If an object, that object becomes the execution context.