Pay attention to the set or the elements of the set in ZF axiomatic system.

◎+= ◎ {◎} = {◎} There is nothing to say. The former equals sign is the definition, and the latter is ◎ ◎ x = x

Natural numbers have never been defined. Here, natural numbers are defined one by one from the empty set, and the natural number set obtained by putting all natural numbers together is the so-called minimum inductive set.

In addition, the set theory in Zoroach's mathematical analysis is not recommended, especially his account of axiom of choice is wrong.