fn add_10(value: Option<u32>) -> Result<u32, String> {
    match value {
        Some(v) => Ok(v + 10),
        None => Err(String::from("value was empty")),
    }
}

fn main() {
    let value = add_10(None);

    println!("Result is: {}", value.unwrap_or(42));

    panic!("Non recoverable error.");
}