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A number of containers express the concept of group of equivalent streams, it cannot be mapped correctly using the concept of Program since even if it signals a group of streams as well, it normally states the group has to be played together.
A MOV signals that only one of the streams belonging to the same group has to be played at the same time.
A group id field will be introduced to AVStream, matching the stream id semantics, thus providing facilities to manipulate and select groups in avconv.