code: func (r FutureSignMessageResult) Receive() (string, error) {
  	res, err := receiveFuture(r)
  	if err != nil {
  		return "", err
  	}
  
  	// Unmarshal result as a string.
  	var b64 string
  	err = json.Unmarshal(res, &b64)
  	if err != nil {
  		return "", err
  	}
  
  	return b64, nil
  }
docs: // Receive waits for the response promised by the future and returns the message
  // signed with the private key of the specified address.