Example 2

All sets consisting of a single element are equipotent, since $\{(a, b)\}$ is the graph of a bijection of $\{a\}$ onto $\{b\}$; in particular, they are all equipotent to $\{\emptyset\}$. The cardinal $\mathrm{Card}(\{\emptyset\}) = \tau_Z(\mathrm{Eq}(\{\emptyset\}, Z))$ is denoted by 1. [^1]