Clarify that walls are circular

main.tex

@@ -375,6 +375,8 @@
 		\item $F$ necessarily semistable
 		\item Polynomial condition on $\alpha, \beta$
 		\item $E$ switches between stable and unstable crossing wall
+		\item For $\sigma_{\alpha, \beta}$: concentric circles on
+			either side of $\beta = \mu(v)$
 	\end{itemize}
 \end{frame}