if let Ok(contents) = std::str::from_utf8(&contents) {
            for l in contents.lines() {
                writeln!(w, "{} {}", pref, l)?;

        if let Ok(mut contents) = std::str::from_utf8(&contents) {
            while let Some(n) = contents.chars().position(|c| c == '\n') {
                let (a, b) = contents.split_at(n + 1);
                contents = b;
                write!(w, "{} {}", pref, a)?;
            }
            if !contents.is_empty() {
                writeln!(w, "{} {}", pref, contents)?;