Function unclaimed 

fn unclaimed(
    root: &Path,
    directory: &str,
    extension: &str,
    claimed: &BTreeSet<String>,
    exempt: &[&str],
) -> Result<(), Error>

Fails when a file on disk appears in no entry.

This is §17.5’s last rejection rule, and the reason the manifest is worth having: adding an example forces a decision about whether it is paired.