use std::convert::TryInto;

fn insert(mut a: Vec<u8>, b: Vec<u8>) -> Result<Vec<u8>, &'static str> {
    if let Ok(l) = TryInto::<u32>::try_into(b.len()) {
        a.extend(&l.to_be_bytes());
        a.extend(b);
        return Ok(a)
    } else {
        Err("4GB vector????")
    }
}