Fern Logistics, a fictional dispatch service, needs utilities that preserve execution order while handling failed work. Some tasks run jobs concurrently; others apply adjustments one at a time. Use the dependency between operations to choose the control flow. The practice also revisits lazy ranges and manual iterator consumption. All jobs and payloads remain local fixtures, so a failure points to your function rather than an outside service. Work through one trace on paper before running it, especially when a callback queues another callback. An empty batch, a rejected job, and a limit of zero are each part of the normal contract.