code: func (b *LRUCache) expiredEntry(now time.Time) *entry {
  	if len(b.priorityQueue) == 0 {
  		return nil
  	}
  
  	if now.IsZero() {
  		// Fill it only when actually used.
  		now = time.Now()
  	}
  
  	if e := b.priorityQueue[0]; e.expire.Before(now) {
  		return e
  	}
  	return nil
  }
docs: // Give me the entry with lowest expiry field if it's before now.