code: func (c *Cache) removeOldest() (key string, value interface{}) {
  	ele := c.ll.Back()
  	if ele == nil {
  		return
  	}
  	c.ll.Remove(ele)
  	ent := ele.Value.(*entry)
  	delete(c.cache, ent.key)
  	return ent.key, ent.value
  
  }
docs: // note: must hold c.mu