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