On is a notation for the class of all ordinals, by the way, which are rich and strange once you go past the finite ones: "first", "second", ... , ω, ω + 1, ... , ω · 2, ω·2 + 1, ... ... ω^ω ... ... ε₀ ... ... -- and those so far are still the countably infinite ones (same size as the whole numbers). Highly recommended mathematical objects, would apply successor operation again. I wish I'd known about them as a kid; they're much more what I wanted than the frustrating "adding one to infinity doesn't do anything" kind that people told me about.