* the most-recently-used bucket of subfacets is kept, so actually an
* arbitrary number of subfacets can be kept in any given expiration run
* (though the next run will delete most of those unless they receive
* the most-recently-used bucket of subfacets is kept, so actually an
* arbitrary number of subfacets can be kept in any given expiration run
* (though the next run will delete most of those unless they receive