 This formula does not mention sethood and has no parameters (or just the set parameter \(\varnothing)\). The class \(V\) of all sets has \(\varnothing\) as a member and contains \(y \cup \{y\}\) if it contains \(y\) by Pairing and Union for sets (already shown). Thus any \(x\) satisfying this formula is a set, whence the extension of the formula is a set (clearly the usual set of von Neumann natural numbers). So Infinity is true in the sets of Ackermann set theory.