#![allow(unused_variables)]
fn main() {
    use std::collections::VecDeque;
    let mut d = VecDeque::new();
    d.push_back(1);
    d.push_back(2);
    assert_eq!(d.pop_front(), Some(1));
    assert_eq!(d.pop_front(), Some(2));
    assert_eq!(d.pop_front(), None);
}