private:
std::list<ContentTimePeriod> image_subtitles_during (ContentTimePeriod, bool starting) const;
std::list<ContentTimePeriod> text_subtitles_during (ContentTimePeriod, bool starting) const;
private:
std::list<ContentTimePeriod> image_subtitles_during (ContentTimePeriod, bool starting) const;
std::list<ContentTimePeriod> text_subtitles_during (ContentTimePeriod, bool starting) const;