- void add_image_subtitle (std::string id, ContentTimePeriod period);
- void add_text_subtitle (std::string id, ContentTimePeriod period);
- std::list<ContentTimePeriod> image_subtitles_during (ContentTimePeriod period, bool starting) const;
- std::list<ContentTimePeriod> text_subtitles_during (ContentTimePeriod period, bool starting) const;
- ContentTime find_subtitle_to (std::string id) const;
- void add_offset (ContentTime offset);