+ /**
+ * Number of chapters in AVChapter array.
+ * When muxing, chapters are normally written in the file header,
+ * so nb_chapters should normally be initialized before write_header
+ * is called. Some muxers (e.g. mov and mkv) can also write chapters
+ * in the trailer. To write chapters in the trailer, nb_chapters
+ * must be zero when write_header is called and non-zero when
+ * write_trailer is called.
+ * muxing : set by user
+ * demuxing: set by libavformat
+ */