<head> and <body>#
Both <head> and <body> are optional.
If there is a <head> element,
it must be the first sub-element of <asdf>.
The <head> element can contain <source> sub-elements
and an optional <reference>.
All elements within <head> exist for the whole duration of the scene.
If they contain transform attributes like pos or rot,
those values are static.
Additional <transform> elements can be used in the <body>
to offset those values dynamically.
The <body> element can contain <seq> and <par> elements,
as well as <clip> and <transform> elements.
If the <body> element contains multiple sub-elements,
it acts like an implicit <seq> element.