- maybe-unfold stop? mapper successor seed ...procedure
- either-unfold stop? mapper successor seed ...procedure
If stop? returns true on seeds, a Nothing / a Left of seeds is returned. Otherwise, successor is applied to seeds. If stop? returns false on the results of successor, it is an error. (This is a condition of kind (exn assertion).) But if the second call to stop? returns true, mapper is applied to seeds and the results are wrapped in a Just/Right and returned.