code: func (whisper *Whisper) Send(envelope *Envelope) error {
  	ok, err := whisper.add(envelope, false)
  	if err == nil && !ok {
  		return fmt.Errorf("failed to add envelope")
  	}
  	return err
  }
docs: // Send injects a message into the whisper send queue, to be distributed in the
  // network in the coming cycles.