protected function doAttach($event, IObserver $observer)
    {
        if (!array_key_exists($event, $this->_observers)) {
            $this->_observers[$event] = array();
        }

        if (in_array($observer, $this->_observers[$event], true)) {
            return;
        }

        array_push($this->_observers[$event], $observer);
    }