plan queues a function

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.