- void add_image_subtitle (std::string id, ContentTimePeriod period);
- void add_text_subtitle (std::string id, ContentTimePeriod period);
- void set_subtitle_to (std::string id, ContentTime to);
- ContentTime find_subtitle_to (std::string id) const;
- bool unknown_to (std::string id) const;
- void add_offset (ContentTime offset);