-- ln(1+x) by x - x^2/2 + x^3/3 - x^4/4 + ...
-- converges -1≤x≤1
--
-- not very good.
λN.λx.
{
  iix ← irange 1 N 1;
  mkIx ← λn. (_((_ x)^n))%(𝑖n);
  (+)/ 0 (mkIx'iix)
}