--- a/slides/slides09.tex	Sat Dec 19 00:21:13 2020 +0000
+++ b/slides/slides09.tex	Fri Jan 22 10:39:04 2021 +0000
@@ -826,7 +826,8 @@
 \end{frame}
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%     
 
-
+\begin{frame}<1-20>[c]
+\end{frame}
 
 \end{document}