code: func bits2octets(in []byte, curve elliptic.Curve, rolen int) []byte {
  	z1 := hashToInt(in, curve)
  	z2 := new(big.Int).Sub(z1, curve.Params().N)
  	if z2.Sign() < 0 {
  		return int2octets(z1, rolen)
  	}
  	return int2octets(z2, rolen)
  }
docs: // https://tools.ietf.org/html/rfc6979#section-2.3.4