code: func (peer *Peer) expire() {
  	unmark := make(map[common.Hash]struct{})
  	peer.known.Each(func(v interface{}) bool {
  		if !peer.host.isEnvelopeCached(v.(common.Hash)) {
  			unmark[v.(common.Hash)] = struct{}{}
  		}
  		return true
  	})
  	// Dump all known but no longer cached
  	for hash := range unmark {
  		peer.known.Remove(hash)
  	}
  }
docs: // expire iterates over all the known envelopes in the host and removes all
  // expired (unknown) ones from the known list.