lib/texinputs/isabelle.sty

 \chardef\isachartilde=`\~
 
 
 % keyword and section markup
 
+\newcommand{\isakeywordcharunderscore}{\_}
 \newcommand{\isakeyword}[1]
-{\emph{\bf\def\isachardot{.}\def\isacharunderscore{-}%
+{\emph{\bf\def\isachardot{.}\def\isacharunderscore{\isakeywordcharunderscore}%
 \def\isacharbraceleft{\{}\def\isacharbraceright{\}}#1}}
 \newcommand{\isacommand}[1]{\isakeyword{#1}}
 
 \newcommand{\isamarkupheader}[1]{\section{#1}}
 \newcommand{\isamarkupchapter}[1]{\chapter{#1}}

 \def\isabellestyle#1{\csname isabellestyle#1\endcsname}
 
 \newcommand{\isabellestyleit}{%
 \renewcommand{\isastyle}{\small\it}%
 \renewcommand{\isastyleminor}{\it}%
+\renewcommand{\isakeywordcharunderscore}{-}%
 %\renewcommand{\isadigit}[1]{\emph{$##1$}}
 \renewcommand{\isacharbang}{\emph{$!$}}%
 \renewcommand{\isachardoublequote}{}%
 \renewcommand{\isacharhash}{\emph{$\#$}}%
 \renewcommand{\isachardollar}{\emph{$\$$}}%