// SPDX-License-Identifier: MIT OR Apache-2.0
struct Node { value: Int, next: Option<Box<Node>> }

fn sum(node: Node) -> Int {
    let rest = match node.next {
        Option.Some(next) => sum(next.into_inner()),
        Option.None => 0,
    };
    return node.value + rest;
}

fn main() {
    let last = Node { value: 20, next: Option<Box<Node>>.None };
    let first = Node { value: 10, next: Option.Some(box(last)) };
    println(sum(first));
}