public function checkPermission()
	{
		if ($this->User->isAdmin)
		{
			return;
		}

		// Show only own undo steps
		$objSteps = $this->Database->prepare("SELECT id FROM tl_undo WHERE pid=?")
								   ->execute($this->User->id);

		// Restrict the list
		$GLOBALS['TL_DCA']['tl_undo']['list']['sorting']['root'] = $objSteps->numRows ? $objSteps->fetchEach('id') : array(0);

		// Redirect if there is an error
		if (Contao\Input::get('act') && !\in_array(Contao\Input::get('id'), $GLOBALS['TL_DCA']['tl_undo']['list']['sorting']['root']))
		{
			throw new Contao\CoreBundle\Exception\AccessDeniedException('Not enough permissions to ' . Contao\Input::get('act') . ' undo step ID ' . Contao\Input::get('id') . '.');
		}
	}