def METHOD_NAME(empty_dag: DAG) -> None:
    """Test threaded walker."""
    dag = empty_dag
    walker = ThreadedWalker(UnlimitedSemaphore())
    # b and c should be executed at the same time.
    dag.from_dict({"a": ["b", "c"], "b": ["d"], "c": ["d"], "d": []})
    lock = threading.Lock()  # Protects nodes from concurrent access
    nodes: List[Any] = []
    def walk_func(node: Any) -> bool:
        with lock:
            nodes.append(node)
        return True
    walker.walk(dag, walk_func)
    assert nodes in [["d", "c", "b", "a"], ["d", "b", "c", "a"]]