def set key, value
      @reverse_cache[value.object_id] = key
      @cache[key]                     = value.object_id
      ObjectSpace.define_finalizer(value, @finalize)
    end
def set key, value
      @reverse_cache[value.object_id] = key
      @cache[key]                     = value.object_id
      ObjectSpace.define_finalizer(value, @finalize)
    end
--
 TODO: Barf if the value.object_id already exists in the cache.