use 0 as integral-value for non-integrable functions, simplify a couple of rewrite rules
% HOL/Multivariate_Analysis/document/root.tex
\documentclass[11pt,a4paper]{article}
\usepackage{graphicx,isabelle,isabellesym,latexsym,textcomp}
\usepackage[only,bigsqcap]{stmaryrd}
\usepackage[utf8]{inputenc}
\usepackage{pdfsetup}
\urlstyle{rm}
\isabellestyle{it}
\pagestyle{myheadings}
\begin{document}
\title{Measure and Probability Theory}
\maketitle
\tableofcontents
\begin{center}
\includegraphics[scale=0.45]{session_graph}
\end{center}
\renewcommand{\isamarkupheader}[1]%
{\section{\isabellecontext: #1}\markright{THEORY~``\isabellecontext''}}
\parindent 0pt\parskip 0.5ex
\input{session}
\end{document}