Let $X$ be a D11: Set such that

(i)	$\mathcal{F}_0, \mathcal{F}_1, \mathcal{F}_2, \ldots \subseteq \mathcal{P}(X)$ are each a D84: Sigma-algebra on $X$

The tail sigma-algebra on $X$ with respect to $\mathcal{F}_0, \mathcal{F}_1, \mathcal{F}_2, \dots$ is the D11: Set \begin{equation} \bigcap_{n \in \mathbb{N}} \sigma \left\langle \bigcup_{m \in \mathbb{N} : m \geq n} \mathcal{F}_m \right\rangle \end{equation}