Let $f$ be a function defined on $R$ by $f(x)=\{\begin{array}{ccc}0& ifxis\text{i}\text{r}\text{r}\text{a}\text{t}\text{i}\text{o}\text{n}\text{a}\text{l}& \\ 1& ifxis\text{r}\text{a}\text{t}\text{i}\text{o}\text{n}\text{a}\text{l}\text{.}& \end{array}$ Then $f(x)=0$ a$.$e$.$ This can equivalently be written as $f=0$ p$.$p$.($presque parpout$).$