A mapping `f : \mathbb{R} \to \mathbb{R}` defined by $ f(x) = \begin{cases} x, & \text{when } x \in \mathbb{Q} \\ 1-x, & \text{when } x \notin \mathbb{Q} \end{cases}$, where `\mathbb{Q}` is the set of rational number. Show that `f \circ f = I_{\mathbb{R}}`