public function addCallback($callback, $driver = null)
	{
		if (!empty($driver)) {
			if (is_array($driver)) {
				if (!in_array($this->index->driver, $driver)) {
					return $this;
				}
			}
			else if ($driver != $this->index->driver) {
				return $this;
			}
		}
		
		$this->callbacks[] = $callback;
		
		return $this;
	}