public function remove_group_permission($group, $action)
	{
		$db =  \App::get('db');
		$all = false;

		if ($group == 'authenticated')
		{
			$group = '0';
		}

		if (!is_numeric($group))
		{
			$query = "SELECT gidNumber FROM `#__xgroups` WHERE cn=" . $db->quote($group) . ";";
			$db->setQuery($query);
			$group_id = $db->loadResult();

			if ($group_id === null)
			{
				$this->_error("Unknown group $group");
				return false;
			}
		}

		$quoted_project_id = $db->quote($this->id);
		$quoted_group_id = $db->quote($group_id);
		$values = '';

		foreach ((array) $action as $a)
		{
			if ($a == '*')
			{
				$all = true;
			}
			if (!empty($values))
			{
				$values .= ',';
			}
			$values .= $db->quote($a);
		}

		$query = "DELETE FROM `#__trac_group_permission` WHERE trac_project_id=$quoted_project_id AND group_id=$quoted_group_id";

		if (!$all)
		{
			$query .= " AND action IN (" .  $values . ");";
		}

		$db->setQuery($query);
		$db->query();
	}