code: func (r FutureGetDifficultyResult) Receive() (float64, error) {
  	res, err := receiveFuture(r)
  	if err != nil {
  		return 0, err
  	}
  
  	// Unmarshal the result as a float64.
  	var difficulty float64
  	err = json.Unmarshal(res, &difficulty)
  	if err != nil {
  		return 0, err
  	}
  	return difficulty, nil
  }
docs: // Receive waits for the response promised by the future and returns the
  // proof-of-work difficulty as a multiple of the minimum difficulty.