Function prune_tables 

fn prune_tables(folder: &Path, sealed: &mut Sealed, used: &BTreeSet<String>)

This computer’s own spam filter’s tables past the newest KEPT_TABLES taken out of the folder now, unless a record of this computer’s names one again (used: a table put back): that one goes by the usual rule. Never another device’s file: only those this computer sealed (mine). A device that reads only the newest record, as every Sioul does, never asks for them; an earlier version of the table kept by reference on another device can no longer be put back from the folder.