fn tail<'a>(s: &'a [u8]) -> &'a [u8] {
      &s[1..] 
    }
    ...
    fn main() -> Result<(), std::str::Utf8Error> {
        let s = std::str::from_utf8(&[240, 159, 141, 137])?;
        println!("{}", s);
        Ok(())
    }