--- a/Slides/document/root.tex	Mon Apr 11 02:25:25 2011 +0100
+++ b/Slides/document/root.tex	Tue Apr 12 15:46:35 2011 +0800
@@ -19,6 +19,7 @@
 \usetikzlibrary{automata}
 \usetikzlibrary{shapes}
 \usetikzlibrary{shadows}
+\usetikzlibrary{positioning}
 %%%\usetikzlibrary{mindmap}
 
 \usepackage{graphicx}