void as_xml (xmlpp::Node *) const;
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);
void as_xml (xmlpp::Node *) const;
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);