fn parse_one(c: char, s: &str) -> Result<u8, &'static str> {
    match c {
        'a' => Ok(0),
        'b' => parse_many(s).map(|m1| parse_many(&s[1..]).map(|m2| 10)),
        _ => Err("Expected a or b"),
    }
}

fn parse_many(s: &str) -> Result<Vec<u8>, &'static str> {
    panic!()
}