code: func (curve *KoblitzCurve) fieldJacobianToBigAffine(x, y, z *fieldVal) (*big.Int, *big.Int) {
  	// Inversions are expensive and both point addition and point doubling
  	// are faster when working with points that have a z value of one.  So,
  	// if the point needs to be converted to affine, go ahead and normalize
  	// the point itself at the same time as the calculation is the same.
  	var zInv, tempZ fieldVal
  	zInv.Set(z).Inverse()   // zInv = Z^-1
  	tempZ.SquareVal(&zInv)  // tempZ = Z^-2
  	x.Mul(&tempZ)           // X = X/Z^2 (mag: 1)
  	y.Mul(tempZ.Mul(&zInv)) // Y = Y/Z^3 (mag: 1)
  	z.SetInt(1)             // Z = 1 (mag: 1)
  
  	// Normalize the x and y values.
  	x.Normalize()
  	y.Normalize()
  
  	// Convert the field values for the now affine point to big.Ints.
  	x3, y3 := new(big.Int), new(big.Int)
  	x3.SetBytes(x.Bytes()[:])
  	y3.SetBytes(y.Bytes()[:])
  	return x3, y3
  }
docs: // fieldJacobianToBigAffine takes a Jacobian point (x, y, z) as field values and
  // converts it to an affine point as big integers.