I also would have thought that it would be covered in a CS curriculum, as I said. it'snot just some random proof, but an important foundational one. (I don't think I "forgot a possibilty", I just thought it usually would).

You are saying you took classes that covered logic, number theory, group theory, algorithmic complexity, discrete math, and calculus, and you are certain none of them covered this? Too bad, that is unfortunate! I'm glad you found it on your own! it's really neat!

Yeah. It's pretty foundational. e.g. https://en.wikipedia.org/wiki/Aleph_number I guess it's possible to teach it without going through the proof.