func exp10(n int) *big.Int {
	if n < len(nat) {
		return nat[n]
	}
	r := big.NewInt(int64(n))
	return r.Exp(natTen, r, nil)
}
func exp10(n int) *big.Int {
	if n < len(nat) {
		return nat[n]
	}
	r := big.NewInt(int64(n))
	return r.Exp(natTen, r, nil)
}
// performance: tested with reference work variable
// - but int.Set is expensive, so let's live with big.Int creation for n >= len(nat)