src/HOL/Library/Library/document/root.tex

 \tableofcontents
 \newpage
 
 \renewcommand{\isamarkupheader}[1]%
 {\section{\isabellecontext: #1}\markright{THEORY~``\isabellecontext''}}
+\renewcommand{\isasymguillemotright}{$\gg$}
 
 \parindent 0pt \parskip 0.5ex
 \input{session}
 
 \pagestyle{headings}