public static function create_grip_channel_header($channels)
    {
        $channels = self::parse_channels($channels);
        $parts = array();
        foreach ($channels as $channel) {
            $s = $channel->name;
            if (!is_null($channel->prev_id)) {
                $s .= "; prev-id={$channel->prev_id}";
            }
            $parts[] = $s;
        }
        return implode(', ', $parts);
    }