fix names in eq. 5.2.2

actions.tex

@@ -346,7 +346,7 @@ \subsection{Transitive $G$-sets}
   it is equivalent to demand this over all $z:\BG$:
   \begin{equation}\label{eq:Gset-trans-gen}
     \prod_{z:\BG}\exists_{x:X(\sh_G)}
-    \prod_{y:X(\sh_G)}\exists_{g:z\eqto z}a=g\cdot b.
+    \prod_{y:X(\sh_G)}\exists_{g:z\eqto z}x=g\cdot y.
   \end{equation}
 
   Yet another equivalent way of expressing that $X$ is transitive is to say