I saw a question where we have $\overline{\mathbb{R}}:=\mathbb{R}\cup\{\pm\infty\}$ and $(X,S)$ is a measurable space, $f:\, X\to\overline{\mathbb{R}}$
In one part I was told to assume that $f$ is measurable as a function $f^{-1}(\mathbb{R})\to\mathbb{R}$
What does it mean ?