#![allow(unused_variables)]
fn main() {
    use std::collections::LinkedList;
    let mut dl = LinkedList::new();
    assert_eq!(dl.back(), None);
    dl.push_back(1);
    assert_eq!(dl.back(), Some(& 1));
    match dl.back_mut() {
        None => {}
        Some(x) => *x = 5,
    }
    assert_eq!(dl.back(), Some(& 5));
}