#![allow(dead_code, deprecated, unused_variables, unused_mut)]
fn is_even(n: &i32) -> bool {
    n % 2 == 0
}
fn main() {
    assert_eq!(None.filter(is_even), None);
    assert_eq!(Some(3).filter(is_even), None);
    assert_eq!(Some(4).filter(is_even), Some(4));
}