fn seven_get_vec() -> BTreeMap<char, BTreeSet<char>> {
    let mut f = File::open("/home/mark/Downloads/input-7").expect("file not found");

    let mut contents = String::new();
    f.read_to_string(&mut contents)
        .expect("something went wrong reading the file");

    let mut output = BTreeMap::new();
    for line in contents.lines() {
        let mut c_i = line.chars();
        for _ in 0.."Step ".len() {
            c_i.next();
        }

        let step_c = c_i.next().unwrap();
        for _ in 0.." must be finished before step ".len() {
            c_i.next();
        }
        let next_step = c_i.next().unwrap();
        let e = output.entry(next_step).or_insert(BTreeSet::new());
        e.insert(step_c);
    }

    output
}