Let $\phi, \psi \in \text{Proposition}$ each be a D2966: Propositional expression.
Then
| (1) | \begin{equation} \models \neg (\phi \lor \psi) \leftrightarrow \neg \phi \land \neg \psi \end{equation} |
| (2) | \begin{equation} \models \neg (\phi \land \psi) \leftrightarrow \neg \phi \lor \neg \psi \end{equation} |