Let $T:E\to F$ be a continuous map between topological spaces. For an ordinal $\beta$ suppose that $\{B_\alpha:\alpha<\beta\}$ is a collection of open subsets of $E$ such that: $\alpha_1<\alpha_2<\beta\Rightarrow B_{\alpha_2}\subset B_{\alpha_1}$ $\bigcap_{\alpha<\beta}B_\alpha\neq\emptyset$ $\...