let mut handler = Handler::new(6);
    if let Err(error) = grab(move |event| {
        handler.callback(event)
    }) {
        println!("Error: {:?}", error)
    }

struct Handler {
    buffer: VecDeque<Key>,
    capacity: usize,
    modifiers: HashSet<Modifier>,
}

impl Handler {
    fn new(capacity: usize) -> Handler {
        Handler {
            buffer: VecDeque::with_capacity(capacity),
            capacity,
            modifiers: HashSet::new(),
        }
    }

    fn callback(&mut self, event: Event) -> Option<Event> {
        // implement here.
    }
}