code: func (rate SatoshiPerByte) ToBtcPerKb() BtcPerKilobyte {
  	// If our rate is the error value, return that.
  	if rate == SatoshiPerByte(-1.0) {
  		return -1.0
  	}
  
  	return BtcPerKilobyte(float64(rate) * bytePerKb * btcPerSatoshi)
  }
docs: // ToBtcPerKb returns a float value that represents the given
  // SatoshiPerByte converted to satoshis per kb.