  Boffa [1988] noticed another consistency proof of NFU. If \(\langle V, \in \rangle\) is a model of \(Z\) (Zermelo set theory) with an external automorphism \(\sigma\) and an ordinal \(\kappa\) such that \(\kappa \rangle \sigma(\kappa)\), then \(V_{\kappa}\) is the domain of a model of NFU whose membership relation is