Are lessons always a combination of Assessables AND Presentations?

[acbart]

Are lessons always a combination of Assessables AND Presentations? Is it possible to have a lesson that had one, but not the other? It seems to me like a Lesson should always have an assessable (otherwise it's not a Lesson, just a Presentation), but it might have no or limited Presentation.

If that definition is accepted, then we could view Exams as Lessons without a Presentation.

https://github.com/acbart/python-sneks/blob/6fef9498ec058ae9bb7f7ef316991f9a947f78ea/docs/terminology.md#L3