The Animation should not directly manipulate a Timeline instance,
but it should defer to the Alpha all handling of the timeline.
This means that:
- set_duration() and set_loop() will either create a Timeline or
will set the :duration and :loop properties on the Timeline; if
the Timeline must be created, and no Alpha instance is available,
then a new Alpha instance will be created as well and the newly
create Timeline will be assigned to the Alpha
- if set_mode() on an Animation instance without an Alpha, the
Alpha will be created; a Timeline will also be created
- set_alpha() will replace the Alpha; if the new Alpha does not
have a Timeline associated then a Timeline will be created using
the current :duration and :loop properties of Animation; otherwise,
if the replaced Alpha had a timeline, the timeline will be
transferred to the new one
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