Add automated tests
Since the project has reached some size and we cannot keep every detail in our minds, we should implement tests for specific usecases. If we make changes, we let these tests run, to see if the change broke anything.
The tests could live in their own directory tests/, similar to demos/ and contain a comparison to the expected behaviour.
A script should be provided to run these tests automatically. Maybe, the could even be run at every commit using CI/CD.