public function addAppender(IdentifierAppenderInterface $appender)
    {
        foreach ((array) $appender->getGroups() as $group_name) {
            if (false === array_key_exists($group_name, $this->appenders)) {
                $this->appenders[$group_name] = array();
            }

            $this->appenders[$group_name][] = $appender;
        }
    }