| context | $X$ |
| definiendum | $\Sigma$ in it |
| postulate | $\Sigma\subseteq \mathcal P(X)$ |
| postulate | $ \Sigma\ne\emptyset $ |
| forall | $E\in\Sigma$ |
| postulate | $ X\smallsetminus E \in \Sigma $ |
| forall | $A\in\mathrm{Sequence}(\Sigma)$ |
| forall | $n\in \mathbb N$ |
| postulate | $ \bigcup_{i=1}^n A_i \in \Sigma $ |
Wikipedia: Sigma-algebra