Task scheduler that pops the earliest deadline once its subtasks are done

Implement a class with add_tasks(tasks) and consume_task(). Each task has an id, a deadline and a list of subtasks.

1. consume_task returns the task with the earliest deadline, or a "no task" result when empty.
2. A task may only be consumed after its subtasks are done; return the earliest-deadline task that is ready.
3. Make both operations fast (for example with a heap) and give their complexity.