Define the BRST transformation of some field \(\phi\) as \(Q \phi\), where under the BRST transformation \(\phi \rightarrow \phi+\delta \phi\) with \(\delta \phi \equiv \theta Q \phi\). For example, from Eq. (9.2.42) we see that \(Q A_{\mu}^{a}=(1 / g) D_{\mu}^{a b} c^{b}\). Show that the BRST variations are nilpotent, which is the statement that \(Q^{2} \phi=0\) when acting on all fields \(\phi\).