fn go() -> Box<Iterator<Item = u32>> {
    let xs = vec![1, 2, 3];
    let iter = (1..5).flat_map(|y| xs.iter().cloned().filter(move |x| x > &y));
    Box::new(iter)
}

fn main() {
    let result = go();
    for i in result {
        println!("{}", i);
    }
}