Function tidy_in 

pub fn tidy_in(
    dir: &Path,
    fit: impl FnOnce(Vec<Queued>) -> Vec<Queued>,
) -> Result<Option<SetAside>>

Fit the waiting memories in dir with fit and set aside the bytes that are not a memory, rewriting the queue only when either changes it. Reading and rewriting under one lock keeps a note queued between them.

Errors

The queue file cannot be rewritten, or the bytes cannot be set aside.