public function size($user = null)
	{
		if ($this->_driver)
		{
			// Use the default user (provided in __construct) if none is given
			if (!$user)
			{
				$user = $this->_user;
			}
			
			$queued = true;
			return $this->_driver->queueLeft($user, $queued);
		}
		
		return null;
	}