macro_rules! maybe {
    ($x:expr) => {
        match $x {
            None => return None,
            Some(x) => x,
        }
    }
}

fn maybe_divide(x: Option<u32>, y: Option<u32>) -> Option<u32> {
    let result = maybe!(x) / maybe!(y);
    Some(result)
}

fn main() {
    let a = maybe_divide(Some(10), Some(2));
    let b = maybe_divide(None, Some(2));
    let c = maybe_divide(Some(10), None);
    let d = maybe_divide(None, None);

    println!("{:?} {:?} {:?} {:?}", a, b, c, d);
}

// Write a macro, `guard`, which takes a condition and returns `None` if the
// condition is not true.