code: func (f *Filter) Trigger(msg *ReceivedMessage) {
  	f.mutex.Lock()
  	defer f.mutex.Unlock()
  
  	if _, exist := f.Messages[msg.EnvelopeHash]; !exist {
  		f.Messages[msg.EnvelopeHash] = msg
  	}
  }
docs: // Trigger adds a yet-unknown message to the filter's list of
  // received messages.