code: func (fs FrontierSigner) SignatureValues(tx *Transaction, sig []byte) (r, s, v *big.Int, err error) {
  	if len(sig) != 65 {
  		panic(fmt.Sprintf("wrong size for signature: got %d, want 65", len(sig)))
  	}
  	r = new(big.Int).SetBytes(sig[:32])
  	s = new(big.Int).SetBytes(sig[32:64])
  	v = new(big.Int).SetBytes([]byte{sig[64] + 27})
  	return r, s, v, nil
  }
docs: // SignatureValues returns signature values. This signature
  // needs to be in the [R || S || V] format where V is 0 or 1.