The seq element specifies a temporal sequence using <seq> tags.
Its children are executed in the order in which they
are listed.
The "begin" values of the children are relative to the end of the child executed
before it, and the "end" values are relative to the beginning of the child
executed before it.
The "abstract" attribute of the seq element gives a brief description of the content contained in the element.
The "author" attribute gives the name of the author of the content contained in the element.
The "begin" attribute specifies the explicit begin time of the element.
Two types of values can be assigned to "begin": a delay-value or an event-value.
A delay-value is a clock value, and the seq element will begin at the specified
time after its parent begins. If the value is an event-value, a specified id
and time is associated with the "begin" attribute. The seq element would then
start at the time given relative to the element of the specified id.
The "copyright" attribute gives copyright information of the content.
The "dur" attribute specifies the explicit duration of the element.
The "end" attribute specifies the explicit end time of the element.
As with "begin," a delay-value or an event-value can be assigned to "end."
The "id" attribute gives a unique id to the element.
"seq" can have the following children: animation, audio, img, ref, text,
textstream, video, a, seq, and par.
