The trick is to not break it up. Adding more lose ends will just make the ball of twine more complicated. What you can break out though is independent code that

The trick is to not break it up. Adding more lose ends will just make the ball of twine more complicated. What you can break out though is independent code that is the same for many apps, that code can be made into a module and reused across apps. Pure mathematical functions can be turned into a library. But what could probably help keep your sanity for a CAS is to add a test case for every change to make sure the same input produce the same output in the future. As well as performance tests to avoid performance regressions.