public function push($element)
    {
        // ask PHP to give me the next index
        // http://stackoverflow.com/q/3698743/2908724
        $this->array[] = 'probe';
        end($this->array);
        $next = key($this->array);
        unset($this->array[$next]);

        // hash that and store
        $this->set($next, $element);

        return $this;
    }