--- a/Slides/document/root.tex	Wed Aug 24 10:01:24 2011 +0000
+++ b/Slides/document/root.tex	Wed Aug 24 22:18:54 2011 +0000
@@ -16,6 +16,7 @@
 \usetikzlibrary{automata}
 \usetikzlibrary{shapes}
 \usetikzlibrary{shadows}
+\usetikzlibrary{calc}
 
 % Isabelle configuration
 %%\urlstyle{rm}