Function check_kept 

async fn check_kept(
    session: &mut Imap,
    account: &Account,
    dir: &Path,
    checker: &Checker,
    host: &str,
    progress: &mut dyn FnMut(&Progress),
    cancel: &Cancel,
    update: &mut Update,
) -> Result<(), Stop>

The messages of an account kept without Sioul’s checks (verify’s work, and every download’s after its new mail), on a session of their own: folder after folder, the newest first, each group’s checks appended to checks.jsonl.gz as they come, so a run stopped goes on there. Their folder gone or renumbered, they are gone: their UIDs name nothing any more.