Let $\theta$ be the unique antiderivative of $\psi$ vanishing at 0. Show that $\theta$ is of class $\mathcal{C}^\infty$, constant on $]-\infty, -1]$ (we denote this constant by $A$) and constant on $[1, +\infty[$ (we denote this constant by $B$). Verify that $A \neq B$.