replaced apply-style proof for instance Multiset :: plus_ac0 by recommended Isar proof style
\documentclass[10pt,a4paper,twoside]{article}
\usepackage{graphicx}
\usepackage{latexsym,theorem}
\usepackage{isabelle,isabellesym}
\usepackage{pdfsetup}\urlstyle{rm}
\begin{document}
\pagestyle{headings}
\pagenumbering{arabic}
\title{The UNITY Formalism}
\author{Sidi Ehmety and Lawrence C. Paulson}
\maketitle
\tableofcontents
\begin{center}
\includegraphics[scale=0.5]{session_graph}
\end{center}
\newpage
\parindent 0pt\parskip 0.5ex
\input{session}
\end{document}