\usepackage{amssymb};

--- a/src/HOL/Library/Library/document/root.tex	Mon Feb 26 23:18:30 2007 +0100
+++ b/src/HOL/Library/Library/document/root.tex	Tue Feb 27 00:32:52 2007 +0100
@@ -4,7 +4,8 @@
 \documentclass[11pt,a4paper]{article}
 \usepackage{ifthen}
 \usepackage[latin1]{inputenc}
-\usepackage{isabelle,isabellesym,pdfsetup}
+\usepackage{isabelle,isabellesym,amssymb}
+\usepackage{pdfsetup}
 
 \urlstyle{rm}
 \isabellestyle{it}