A D11: Set $x$ is an inductive set if and only if
(1) | \begin{equation} \emptyset \in x \end{equation} |
(2) | \begin{equation} \forall \, y \in x : y \cup \{ y \} \in x \end{equation} |
(1) | \begin{equation} \emptyset \in x \end{equation} |
(2) | \begin{equation} \forall \, y \in x : y \cup \{ y \} \in x \end{equation} |