Self-typed fluent API

Inside Solution, write: 1. abstract static class Node<Self extends Node<Self>> with an INSTANCE-level List<String> labels and public Self label(String l) that records the label and returns (Self) this (annotated @SuppressWarnings("unchecked")). 2. static class Task extends Node<Task> with public String describe() returning "Task" + labels (e.g. Task[a, b]). 3. static class Job extends Node<Job> with describe() prefixing "Job". Instances must NOT share state: labeling one object must never appear in another's describe().

Difficulty: advanced