    protected function filter($tokens, $config, $context) {
        $allowed = $config->get('Attr.AllowedClasses');
        $forbidden = $config->get('Attr.ForbiddenClasses');
        $ret = array();
        foreach ($tokens as $token) {
            if (
                ($allowed === null || isset($allowed[$token])) &&
                !isset($forbidden[$token]) &&
                // We need this O(n) check because of PHP's array
                // implementation that casts -0 to 0.
                !in_array($token, $ret, true)
            ) {
                $ret[] = $token;
            }
        }
        return $ret;
    }
The software does not neutralize or incorrectly neutralizes user-controllable input before it is placed in output that is used as a web page that is served to other users.