r.prevRune = -1
	var abs int64
	switch whence {
	case io.SeekStart:
		abs = offset
	case io.SeekCurrent:
		abs = r.i + offset
	case io.SeekEnd:
		abs = int64(len(r.s)) + offset
	default:
		return 0, errors.New("bytes.Reader.Seek: invalid whence")
	}
	if abs < 0 {
		return 0, errors.New("bytes.Reader.Seek: negative position")
	}
	r.i = abs
	return abs, nil
}

func OverlayDir(dstRoot, srcRoot string) error {
	dstRoot = filepath.Clean(dstRoot)
	if err := os.MkdirAll(dstRoot, 0777); err != nil {
		return err
	}

	srcRoot, err := filepath.Abs(srcRoot)
	if err != nil {
		return err
	}

	return filepath.Walk(srcRoot, func(srcPath string, info os.FileInfo, err error) error {
		if err != nil || srcPath == srcRoot {
			return err
		}

// Neg sets z = -x and returns z.
func (z *Int) Neg(x *Int) *Int {
	x.doinit()
	z.doinit()
	C.mpz_neg(&z.i[0], &x.i[0])
	return z
}

// Abs sets z to the absolute value of x and returns z.
func (z *Int) Abs(x *Int) *Int {
	x.doinit()
	z.doinit()
	C.mpz_abs(&z.i[0], &x.i[0])
	return z
}

/*
 * functions without a clear receiver
 */

// CmpInt compares x and y. The result is
//