pub fn make_tm(tm_desc_file: &str) -> Result<TM, Box<Error>> {

    ...

    match iter.next() {
        Some("alphabet") => (),
        _                => return Result::Err(Box::from(io::Error::new(
                                io::ErrorKind::InvalidData,
                                format!("Error parsing TM description on line {}", state_count + 2)))),
    }

    ...

}