but that's exactly the case where things become an issue. if you were able to write a library, it would be worth the effort to write it once more from scratch and make sure that the copyright of the previous code is not infringed.

instead however, what happens is that you have a problem that looks similar to a previous problem, so you take the old code and modify it to fit the new problem.

NOW you have the additional problem of having reused code that's owned by employer A, for a project at employer B.