Every element has an animation queue.
When a program calls element animation methods
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.
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.