Example 3

If we take $S\{u\}$ to be the term $\mathfrak{P}(u)$, and $a$ to be a set E, it follows likewise that there exists a mapping, denoted by $n \to \mathfrak{P}^n(\mathrm{E})$, of $\mathbf{N}$ into a set V(E) such that $\mathfrak{P}^0(\mathrm{E}) = \mathrm{E}$, $\mathfrak{P}^1(\mathrm{E}) = \mathfrak{P}(\mathrm{E})$, and $\mathfrak{P}^{n+1}(\mathrm{E}) = \mathfrak{P}(\mathfrak{P}^n(\mathrm{E}))$ for every integer $n$.