Let $\varphi$ and $\psi$ be formulas in propositional logic. We say
that $\varphi \models \psi$ if and only if every model $\mathcal{M}$ that satisfies
$\varphi$ ($\mathcal{M} \models \varphi$) also satisfies $\psi$ ($\mathcal{M} \models \psi$).