code: func (c *rankCache) IDs() []uint64 {
  	c.mu.Lock()
  	defer c.mu.Unlock()
  	a := make([]uint64, 0, len(c.entries))
  	for id := range c.entries {
  		a = append(a, id)
  	}
  	sort.Sort(uint64Slice(a))
  	return a
  }
docs: // IDs returns a list of all IDs in the cache.