Hi Marco, maybe I should make it more clear: when I’m talking about “sets” I’m talking about the set in the most ABSTRACT way. Think of it like a synonym for “group”. In this article I wanted to avoid CT at all costs since it’s such a huge barrier to entry in our field (programmers). For this, I just skipped the second part of the sentence, the one about “category” and “endofuncors”. I’ll make a little edit to my post :)