Hey mzez.

You can do what you are thinking of and just for some general u, then replace ln(1 + u) = f(u) with some general u = g(x).

The only thing though that you need to be careful of is that for a general u = g(x) is that the function is defined for that particular g(x). If it isn't defined and isn't analytic around that point, then you can't use that function.

Otherwise if it's analytic and converges around that particular point, then it should be OK.