code: func (rate SatoshiPerByte) Fee(size uint32) btcutil.Amount {
  	// If our rate is the error value, return that.
  	if rate == SatoshiPerByte(-1) {
  		return btcutil.Amount(-1)
  	}
  
  	return btcutil.Amount(float64(rate) * float64(size))
  }
docs: // Fee returns the fee for a transaction of a given size for
  // the given fee rate.