from __future__ import annotations


class Node:
    def __init__(self, value: int, child: Node | None = None):
        self.value = value
        self.child = child


def flatten(node: Node | None) -> list[int]:
    values: list[int] = []
    while node is not None:
        values.append(node.value)
        node = node.child
    return values


def run() -> None:
    chain = Node(1, Node(2, Node(3)))
    print('annotations:', flatten(chain))
    print('future:', flatten.__annotations__)


if __name__ == '__main__':
    run()