--- a/document/root.tex	Thu Jan 10 13:13:27 2013 +0000
+++ b/document/root.tex	Fri Jan 11 05:45:40 2013 +0000
@@ -3,6 +3,7 @@
 \usepackage{isabellesym}
 \usepackage{times}
 \usepackage{amssymb}
+\usepackage{amsmath}
 \usepackage{mathpartir}
 \usepackage{pdfsetup}
 \usepackage{tikz}