States, that equality is a reflexive relation. Note that equality is defined on the entire class universe. This differs from the diagonal relation E = {t | ∃x. t = (x, x)}. Firstly, E is only defined on the universal class, i.e. only for sets, but not for proper classes. Secondly, one has to write (x, x) ∈ E instead of E x x, since E is a class, not a binary relation symbol. The syntax x = x, on the other hand, is a shorthand for eq x x.