Simple aging lookup table. When a member is accessed it's
 moved to the top of the list and when there are too many elements
 the least-recently-accessed element falls of the end.

  - areSimilar: Keep at most once association for two similar keys (as given by areSimilar)

 Simple priority caching for a small number of key/value associations.
 This cache may age-out results that have been Set by the caller.
 Because of this, the caller must be able to tolerate values
 that aren't what was originally passed to the Set function.

 Concurrency: This collection is thread-safe, though concurrent use may result in different
 threads seeing different live sets of cached items.

  - areSimilar: Keep at most once association for two similar keys (as given by areSimilar)