Gödel numbering
The stage = Def () uses formulas with parameters from to define the sets and . If one discounts (for the moment) the parameters, the formulas can be given a standard Gödel numbering by the natural numbers. If is the formula with the smallest Gödel number that can be used to define , and is the formula with the smallest Gödel number that can be used to define , and is different from , then let &lt; if and only if &lt; in the Gödel numbering. Henceforth, we suppose that = .