------------------+
            |    |    |trun|    |    |    |    |    | Track Fragment Run Box       |
            +----+----+----+----+----+----+----+----+------------------------------+
            |    |    |sgpd|*   |    |    |    |    | Sample Group Description Box |
            +----+----+----+----+----+----+----+----+------------------------------+
            |    |    |sbgp|    |    |    |    |    | Sample to Group Box          |
            +----+----+----+----+----+----+----+----+--------------