code: def _ref_key(self, key):
          
          queue = self.queue
          refcount = self.refcount
  
          queue.append(key)
          refcount[key] = refcount[key] + 1
  
          # periodically compact the queue by eliminating duplicate keys
          # while preserving order of most recent access.  Note that this
          # is only required when the cache does not exceed its maximum
          # size
          if len(queue) > self.max_queue:
              refcount.clear()
              queue_appendleft = queue.appendleft
              queue_appendleft(self.sentinel)
              for k in filterfalse(refcount.__contains__,
                                   iter(queue.pop, self.sentinel)):
                  queue_appendleft(k)
                  refcount[k] = 1
docs: Record a reference to the argument key.