code: func (c *TwoQueueCache) Keys() []interface{} {
  	c.lock.RLock()
  	defer c.lock.RUnlock()
  	k1 := c.frequent.Keys()
  	k2 := c.recent.Keys()
  	return append(k1, k2...)
  }
docs: // Keys returns a slice of the keys in the cache.
  // The frequently used keys are first in the returned slice.