fn find_files(mut img: Box<dyn Read>, outp: Box<dyn Write>, len: Option<u64>) -> Result<()> {
    let mut ser = postcard::Serializer {
        output: WriteFlavor::new(outp)
    };
    let mut seq = ser.serialize_seq(None)?; // or None if unknown

    let mut pb = len.map(|l| ProgressBar::new(l));
    for i in xfind::find_iter(b"FILE0", &mut img) {
        let i = i?;
        seq.serialize_element(&i)?;

        match pb.as_mut() {
            Some(p) => p.set_position(i as u64),
            None => {}
        }
    }    

    seq.end()?;

    Ok(())
}