One way to extend this exercise would be to add a special symbol `'#'`,
and then the task would be to copy from the target `'S'` only back to
the closest `'#'`. This would allow our task to simulate delimited
-continuations with embedded prompts.
+continuations with embedded `prompt`s (also called `reset`s).
The reason the task is well-suited to the list zipper is in part
because the list monad has an intimate connection with continuations.