Every element has an animation queue.
When a program calls element animation methods
such as fd or rt
or dot or play,
the animation does not execute
immediately. Instead, the action is added to the
element's animation queue, and the animations are
executed over time, in sequence.
The plan method adds some code to be run as part
of the animation queue:
fd 100 plan -> write 'moved' bk 100
The function passed to plan will run in sequence,
at its turn in the queue.