code: func (s *Sync) Missing(max int) []common.Hash {
  	var requests []common.Hash
  	for !s.queue.Empty() && (max == 0 || len(requests) < max) {
  		requests = append(requests, s.queue.PopItem().(common.Hash))
  	}
  	return requests
  }
docs: // Missing retrieves the known missing nodes from the trie for retrieval.