code: func (api *PublicWhisperAPI) GetFilterMessages(id string) ([]*Message, error) {
  	api.mu.Lock()
  	f := api.w.GetFilter(id)
  	if f == nil {
  		api.mu.Unlock()
  		return nil, fmt.Errorf("filter not found")
  	}
  	api.lastUsed[id] = time.Now()
  	api.mu.Unlock()
  
  	receivedMessages := f.Retrieve()
  	messages := make([]*Message, 0, len(receivedMessages))
  	for _, msg := range receivedMessages {
  		messages = append(messages, ToWhisperMessage(msg))
  	}
  
  	return messages, nil
  }
docs: // GetFilterMessages returns the messages that match the filter criteria and
  // are received between the last poll and now.