code: func (b *LRUCache) removeEntry(e *entry) {
  	if e.element.list != &b.lruList {
  		panic("list lruList")
  	}
  
  	if e.index != -1 {
  		heap.Remove(&b.priorityQueue, e.index)
  	}
  	b.lruList.Remove(&e.element)
  	b.freeList.PushElementFront(&e.element)
  	delete(b.table, e.key)
  	e.key = ""
  	e.value = nil
  }
docs: // Move entry from used/lru list to a free list. Clear the entry as well.