fn main() {
    let five = Option::Some(5);
    let six = plus_one(&five);
    let none = plus_one(&Option::None);

    println!("five: {:?}", five);
    println!("six: {:?}", six);
    println!("none: {:?}", none);
}

#[derive(Debug)]
enum Option<T> {
    Some(T),
    None,
}

fn plus_one(x: &Option<i32>) -> Option<i32> {
    match x {
        Option::None => Option::None,
        Option::Some(i) => Option::Some(i + 1),
    }
}