#![allow(dead_code, deprecated, unused_variables, unused_mut)]
use std::collections::BTreeMap;
fn main() {
    let mut bt = BTreeMap::new();
    bt.insert(20u8, "foo");
    bt.insert(42u8, "bar");
    let res = [0u8, 1, 11, 200, 22]
        .into_iter()
        .map(|x| {
            x.checked_sub(1)
                .and_then(|x| x.checked_mul(2))
                .and_then(|x| bt.get(&x))
                .or(Some(&"error!"))
                .copied()
                .unwrap()
        })
        .collect::<Vec<_>>();
    assert_eq!(res, ["error!", "error!", "foo", "error!", "bar"]);
}