Third attempt

In this attempt we proceed as in the second, but using lazy evaluation. But, as it turns out, if there is an improvement, it is imperceptible. The reason is more or less obvious. In the usual algorithms, which are the ones that we implemented, least significant digits are computed first. Thus, even if one wants only one significative digit of fⁿ(x₀), more than 2ⁿ digits have to be computed first.