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;
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;