  \((\varrho\) is set-theoretic rank.) Remarkably, as Holmes showed, one can even recover from this model of NFU the nonstandard model of ZF that gave rise to it. He does this by exploiting the idea of set pictures. We saw earlier the idea that set theory is the theory of one extensional binary relation (with equality of course: extensionality cannot be defined in a language without equality). Thus one could say that a set (or, better, a set picture) is a isomorphism class of binary structures consisting of a domain, an extensional binary relation, and a designated element of the carrier set. Relational structures of this kind look like transitive closures of sets and the designated element of the domain indicates the set whose transitive closure is being depicted. (Consequently there is an accessibility condition for the designated element: if one thinks of the extensional relation as a graph, there must be a directed path from any element to the designated element).